Boris Johnson has refused to deny using his personal email address to carry out government business amid concerns about the conduct of a sacked cabinet minister.
Asked by reporters during a campaign visit to the town of Batley whether he had had also broken the rules, Mr Johnson said would not comment on the matter.
It comes after Labour demanded an investigation into allegations that Matt Hancock and junior health minister Lord Bethell had used their personal accounts for official work.
During the by-election campaign trail visit to Johnstone's Paints Limited, the Prime Minister said: "I don't comment on how I conduct Government business."
Justice Secretary Robert Buckland this morning admitted the practice of using personal email accounts was a threat to government security.
Recommended Sajid Javid says camera that caught Matt Hancock’s affair has been ‘disabled’ Sajid Javid rules out further lockdowns: ‘There’s no going back’ Inside Politics: Whitehall launches camera probe after Hancock’s candid clip
Asked in a Monday morning interview whether Matt Hancock's use of a personal email account was a "huge security issue" that could potentially see hackers gain access to government communications, Justice Secretary Robert Buckland told the BBC:
"I agree, and that's why I think it's important that we use the systems that we are provided with."
Ministers are supposed to use their secure government emails and telephones for work to avoid hackers and foreign security services listening in on the inner workings of government.
The approach also ensures official correspondence is logged and subject to data protection and freedom of information laws.
Questions have been raised about whether Mr Hancock’s use of a personal email account will make it harder to scrutinise the process by which so many emergency Covid contracts came to be given to Conservative aquaintances and donors.
But commenting on the admission about security by Mr Buckland, Angela Rayner, Labour's deputy leader, said: “It’s staggering that a government minister has admitted that the Tory party could be putting national security at risk by carrying out government business on private emails but hasn't said that ministers are going to do anything about it.
"We already know that hostile actors target Ministers' private email accounts to access sensitive information.
"We need a full independent inquiry to get to the bottom of how wide this goes, whether Ministers have put our national security at risk and what steps will be taken to protect vital information and our country’s security.”
Mr Johnson’s visit to Batley comes ahead of the Batley and Spen by election, scheduled for 1 July, where the Tories are hoping to win a seat off Labour.
Speaking on Monday morning less than 48 hours after Mr Hancock's resignation, Mr Buckland claimed that under the rules, "anything that is very sensitive, I can assure viewers, isn't actually viewed on email".
UK news in pictures Show all 50
1/50UK news in pictures
UK news in pictures SEI84932542.jpg People walk along Regent Street in central London during a #FreedomToDance march organised by Save Our Scene, in protest against the government’s perceived disregard for the live music industry throughout the coronavirus pandemic
PA
UK news in pictures 26 June 2021 A pair of marchers in a Trans Pride rally share a smile in Soho
Angela Christofilou/The Independent
UK news in pictures 25 June 2021 Tim Duckworth during the Long Jump in the decathlon during day one of the Muller British Athletics Championships at Manchester Regional Arena
PA
UK news in pictures 24 June 2021 A member of staff poses with the work 'The Death of Cash' by XCopy at the 'CryptOGs: The Pioneers of NFT Art' auction at Bonhams auction house in London
EPA
UK news in pictures 23 June 2021 Bank of England Chief Cashier Sarah John displays the new 50-pound banknote at Daunt Books in London
Bank of England via Reuters
UK news in pictures 22 June 2021 Actor Isaac Hampstead Wright sits on the newly unveiled Game of Throne's "Iron Throne" statue, in Leicester Square, in London, Tuesday, June 22, 2021. The statue is the tenth to join the trail and commemorates 10 years since the TV show first aired, as well as in anticipation for HBO's release of House of the Dragon set to be released in 2022
AP
UK news in pictures 21 June 2021 Scotland's First Minister Nicola Sturgeon receives her second dose of the Oxford/AstraZeneca Covid-19 vaccine
AFP/Getty
UK news in pictures 20 June 2021 Joyce Paton, from Peterhead, on one of the remaining snow patches on Meall a’Bhuiridh in Glencoe during the Midsummer Ski. The event, organised by the Glencoe Mountain Resort, is held every year on the weekend closest to the Summer Solstice
PA
UK news in pictures 19 June 2021 England appeal LBW during day four of their Women’s International Test match against India at the Bristol County Ground
PA
UK news in pictures 18 June 2021 Scotland fans let off flares in Leicester Square after Scotland's Euro 2020 match against England ended in a 0-0 draw
Getty
UK news in pictures 17 June 2021 Members of the Tootsie Rollers jazz band pose on the third day of the Royal Ascot horse racing meet
AFP/Getty
UK news in pictures 16 June 2021 A woman and child examine life-size sculptures of a herd of Asian elephants set up by the Elephant Family and The Real Elephant Collective to help educate the public on the elephants and the ways in which humans can better protect the planets biodiversity, in Green Park, central London
AFP/Getty
UK news in pictures 15 June 2021 Hydrotherapists with Dixie, a seven-year-old Dachshund who is being treated for back problems common with the breed, in the hydrotherapy pool during a facility at Battersea Dogs and Cats Home's in Battersea, London, to view their new hydrotherapy centre
PA
UK news in pictures 14 June 2021 Scotland's David Marshall in the net after Czech Republic's Patrik Schick scored their second goal at Hampden Park
Reuters
UK news in pictures 13 June 2021 Raheem Sterling celebrates with Harry Kane after scoring England’s first goal of the Euro 2021 tournament in a match against Croatia at Wembley
Reuters
UK news in pictures 12 June 2021 Oxfam campaigners wearing costumes depicting G7 leaders pose for photographers on Swanpool Beach near Falmouth, Cornwall
EPA
UK news in pictures 11 June 2021 Members of the Vaxinol team, who are commercial, industrial and residential cleaners specialising in disinfection and decontamination, use electrostatic spray systems to deep clean the Only Fools Bar in Liverpool
PA
UK news in pictures 10 June 2021 A woman walks her dogs as the incoming tide begins to wash away the heads of G7 leaders drawn in the sand by activists on the beach at Newquay, Cornwall
AP
UK news in pictures 9 June 2021 Adam Chamberlain, 45, general manager of Big Tree pub in Sheffield, has put up over 500 flags, taking 36 hours, in preparation for Euro 2020, which kicks off this weekend
Tom Maddick / SWNS
UK news in pictures 8 June 2021
REUTERS
UK news in pictures 7 June 2021 A pedestrian wearing a face covering walks over Westminster Bridge near the Houses of Parliament in central London
AFP/Getty
UK news in pictures 6 June 2021 Isobel Salamon, founder of the Edinburgh Cinema Club, poses alongside the Leith Trainspotting murals in Quality Yard, Leith, Edinburgh, for the programme launch of the Cinescapes Festival which starts on July 4 with a Trainspotting 1 and 2 double bill
PA
UK news in pictures 5 June 2021 A long exposure photograph captures the rotation of the earth as the stars blur into circles over Knowlton church ruins in Dorset
Nick Lucas/SWNS
UK news in pictures 4 June 2021 Balloonists take flight during the opening of the Midlands Air Festival in Alcester, Warwickshire
PA
UK news in pictures 3 June 2021 Members of the Household Cavalry during the Major General's annual inspection of the Household Cavalry Mounted Regiment in Hyde Park, London
PA
UK news in pictures 2 June 2021 Hannah Vitos of the Blenheim Art Foundation, poses for a photograph next to artist Ai Weiwei's Gilded Cage (2017) sculpture in the grounds of Blenheim Palace in Woodstock, Britain
Reuters
UK news in pictures 1 June 2021 People swim in the Sky Pool, a transparent swimming pool bridge across two exclusive residential blocks standing next to the US Embassy in Nine Elms, in London, Tuesday, June 1, 2021
AP
UK news in pictures 31 May 2021 People enjoy the hot weather at Brighton beach
Reuters
UK news in pictures 30 May 2021 People venture into the sea as they enjoy themselves during a hot day on Brighton Beach
AP
UK news in pictures 29 May 2021 Swimmers at the Stonehaven Open Air Pool in Aberdeenshire, which reopens after lockdown restrictions were eased
PA
UK news in pictures 28 May 2021 Britain's Prime Minister Boris Johnson gestures as he meets Hungary's Prime Minister Viktor Orban at Downing Street in London
REUTERS
UK news in pictures 27 May 2021 White Pelicans in the sunshine in St James's Park, London
PA
UK news in pictures 26 May 2021 Boats are seen at Southsea Moorings in Portsmouth
Reuters
UK news in pictures 25 May 2021 York Glaziers Trust employees Kieran Muir (left) and Emily Price (right) remove a stained glass window panel at the start of a new five year, ￡5m project to conserve York Minster’s South East Transept and its medieval St Cuthbert Window
PA
UK news in pictures 24 May 2021 Dark rain clouds above an oast house at Bewl Water reservoir near Lamberhurst in Kent during one of the rainiest Mays on record, with the UK seeing 131 per cent of the usual month’s rainfall already
PA
UK news in pictures 23 May 2021 The Premier League trophy with the Manchester City club colour ribbons on, at Etihad Stadium, prior to the last Premier League match of the season. City will finally pick up the trophy after they won the league on 11 May
Getty
UK news in pictures 22 May 2021 Gary Kenny lifts the Buildbase FA Vase Trophy after Warrington Rylands won the FA Vase Final against Binfield at Wembley Stadium
Getty
UK news in pictures 21 May 2021 A family buffeted by the wind whilst crossing the the Millennium Bridge in London, with wind and rain forecast to ravage the UK on the first Friday that people have been allowed to meet in large groups outside in England
PA
UK news in pictures 20 May 2021 Devon And Cornwall Police Demonstrate Their Skills For Policing The G7 Summit
Getty Images
UK news in pictures 18 May 2021 An employee stands before a costume for the Queen of Hearts by Bob Crowley on display at the Alice: Curiouser and Curiouser exhibition at the Victoria and Albert Museum in London
PA
UK news in pictures 17 May 2021 Passengers prepare to board an easyJet flight to Faro, Portugal, at Gatwick Airport after the ban on international leisure travel for people in England was lifted following the further easing of lockdown restrictions in England
PA
UK news in pictures 16 May 2021 Emergency workers at the scene of a suspected gas explosion, in which a young child was killed and two people were seriously injured, on Mallowdale Ave Heysham which caused 2 houses to collapse and badly damaged another
PA
UK news in pictures 15 May 2021 Pro-Palestinian activists and supporters let off smoke flares, wave flags and carry placards during a demonstration in support of the Palestinian cause as violence escalates in the ongoing conflict with Israel, in central London
AFP via Getty
UK news in pictures 14 May 2021 Member of staffs tighten screws and paint a Marlin skeleton, before it goes on display at the Natural History Museum in London, as the museum prepares to reopen to the public on 17 May, following the further easing of lockdown restrictions in England
PA
UK news in pictures 13 May 2021 A worshipper at the Baitul Futuh Mosque in Mordon, south London, ahead of Eid al-Fitr. The celebration marks the end of the Muslim month of fasting, called Ramadan.
PA
UK news in pictures 12 May 2021 A couple have wedding photos taken in Westminster, London
Getty
UK news in pictures 11 May 2021 The sun rises on Coquet Island, off Amble on the Northumberland coast, where as many as 35000 seabirds cram onto this tiny island to breed
PA
UK news in pictures 10 May 2021 Newly elected for a second term Mayor of London Sadiq Khan during his signing in ceremony at Shakespeare's Globe Theatre on London’s Southbank
PA
UK news in pictures 9 May 2021 People mill around St. Michael's tower on top of Glastonbury Tor as it is seen through blooming yellow rapeseed on a day of mixed weather in Glastonbury, Somerset
PA
UK news in pictures 8 May 2021 Wales First Minister Mark Drakeford elbow bumps newly elected MS Labour candidates Elizabeth Buffy Williams, Rhondda, left, and Sarah Murphy, Bridgend & Porthcawl Labour, right, as they meet in Porthcawl, Wales
PA
On the question of an inquiry, he told Sky News: "We should use Government emails, I think that's very clear.
"I think the Cabinet Office, if they're asked to look at this, they probably will be, will need to satisfy themselves that if that was the case then the material is available."
Mr Hancock quit on Saturday following revelations about his relationship with an aide and CCTV footage of them embracing and apparently breaking social distancing instructions in a government office.