Metropolitan Police officers have begun interviewing key witnesses in the “partygate” scandal which rocked Boris Johnson’s premiership – seven weeks on from start of the investigation.

       Scotland Yard announced on Monday that over 100 questionnaires have been sent out to Downing Street and Whitehall staff asking them about alleged Covid lockdown gatherings.

       “In addition to the detailed review of all available material, including returned questionnaires, detectives from the Operation Hillman investigation team have started interviewing key witnesses,” said a Met police spokesperson.

       Mr Johnson was urged to “come clean” on whether he was one of the “key witnesses”, since neither No 10 nor the police would say whether the PM had been interviewed by officers.

       Liberal Democrat leader Sir Ed Davey said: “Johnson must come clean: is he being interviewed by the police, and will he resign if found to have broken the law? No more fluff, just the facts for all those who suffered during lockdown while he partied.”

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       The Met police revealed that no fines had been handed out so far, with no referrals to the ACRO Criminal Records Office for the issuing of Fixed Penalty Notices made at this stage.

       “However, every questionnaire response is being assessed alongside all available evidence, and should this reach the evidential threshold, then referrals will be made,” the spokesperson said.

       But investigators have identified more individuals on the basis on questionnaire responses, warned that more staff may be contacted and asked to explain their actions as new details come to light.

       A special Scotland Yard team is investigating 12 government gatherings as part of Operation Hillman, including as many as six which Mr Johnson is reported to have attended.

       No 10 confirmed that Mr Johnson had returned his document to investigating officers during the third week of February, having received it the previous week.

       Scotland Yard did not reveal why it was taking so long to work through the partygate probe. The force said each witnesses should have completed and returned their questionnaires within seven days of receiving it.

       “This investigation involves a significant amount of investigative material; the serving of over 100 questionnaires and the need to individually assess every response,” said a spokesperson. “We are progressing the investigation as quickly as possible.”

       The statement added: “As a result of responses so far, further individuals have been identified and questionnaires sent to them. As the investigation continues, we may need to contact more people as further information comes to light.”

       It comes as one of the country’s top polling experts said the British public are willing to overlook Mr Johnson’s recent scandals and back him again at the upcoming local elections.

       Lord Hayward said the prime minister had “a hold” on the electorate – saying Mr Johnson had the ability to defy terrible personal approval ratings at the May elections.

       Asked if many voters would vote for a Tory party led by Mr Johnson, even if they don’t like Mr Johnson personally, the tory peer said: “Yes. He has a hold on people … like no other British politician in my lifetime.”

       Lord Hayward added: “Things Boris has done or said impacts less on people’s willingness to support him than other politicians.”

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       Cabinet minster Jacob Rees-Mogg recently waved away concerns over parties in No 10 during lockdown as “fluff” and “fundamentally trivial”.

       Scottish Tory leader Douglas Ross and backbench MP Andrew Bridgen are among the ex-rebels to have withdrawn letters of no confidence in the prime minister due to the seriousness of the Ukraine crisis.

       Asked if his U-turn made him look a “lightweight”, Mr Ross said “I don’t care … [Partygate] really does seem so small in comparison to a country defending itself against atrocious actions from the Russians and Vladimir Putin.”