Brexit minister David Frost has reportedly resigned from Boris Johnson’s cabinet, triggering a fresh crisis inside Downing Street after an already turbulent week for the prime minister.

       Lord Frost — one of the most popular members of the cabinet among the Conservative faithful — helped negotiate the Brexit agreement and in recent months has been instrumental in negotiations with the EU over the Northern Ireland Protocol.

       According to the Mail on Sunday, the senior minister, who Mr Johnson had elevated to the Lords, handed in his resignation a week ago, but has been persuaded to stay in his post until January.

       The newspaper quoted sources saying his departure had been prompted by the implementation of the Covid “plan B” measures earlier this week which promoted a 100-strong rebellion among Conservative MP — the biggest revolt of Mr Johnson’s premiership.

       However, it was added Lord Frost was also concerned over the government’s decision to hike national insurance from April.

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       In a recent speech to the Centre of Policy Studies, the Brexit minister said he agreed with Rishi Sunak, the chancellor, that the Conservatives’ “goal must be to reduce taxes” and said if Brexit is to succeed if “all we do is import the European social model”.

       He was also effusive in his praise of the lack of Covid restrictions during the 23 November speech, saying: “Unavoidably, we have had a lot of state direction and control during the pandemic.

       “That cannot and must not last for ever, and I am glad that it is not. I am very happy that free Britain, or at least merry England, is probably now the free-est country in the world as regards covid restrictions. No mask rules, no vaccine passports - and long may it remain so.”

       His resignation represents a major political blow from the prime minister who is already facing a series of crises over “gatherings” in No 10 during Covid restrictions, growing discontent on the Tory backbenches over his leadership and yesterday’s historic loss in the North Shropshire by-election — a seat held by the party since 1832.

       Just yesterday, Tory MPs put Mr Johnson on notice while one backbencher revealed he had submitted a letter of no confidence in the prime minister to the chair of the 1922 committee of backbench Conservatives MP.

       Jenny Chapman, the shadow Brexit secretary, said: “As if we didn’t already know, Lord Frost resigning shows the government’s in chaos.

       “The country needs leadership not a lame duck PM whose MPs and cabinet have lost faith in him. Boris Johnson needs to apologise to the public and explain what his plan is for the next few weeks.”

       The Liberal Democrat foreign affairs spokesperson, Layla Moran, added: “This shock resignation is a sign of the chaos and confusion at the heart of this Conservative government. The rats are fleeing Boris Johnson’s sinking ship as he lurches from crisis to crisis.

       “Even the prime minister’s once-loyal supporters are now abandoning him, just as lifelong Conservative voters are switching in their droves to the Liberal Democrats.

       “At a time we need strong leadership to get us through the pandemic, we instead have a weak Prime Minister who has lost the support of his allies and the trust of the British people.”

       On Friday, it had also emerged the UK had abandoned its attempt to strip EU judges of the power to oversee the Northern Ireland Protocol — despite repeated pledges by Lord Frost and the government to “remove” the role of the European Court of Justice (ECJ).

       In October, Lord Frost travelled to Lisbon to vow the ECJ would not be allowed to have a remit, but new UK proposals would see it interpret matters of EU law.

       Disputes would be settled by an independent arbitration panel, rather than the European Commission, a model offered to Brussels by Switzerland. At a Brussels press conference, Maros Sefcovic, the commission vice-president, said the UK signed up to the ECJ’s existing remit, so it was “a topic we are not ready to include in our discussions”.

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       Downing Street did not immediately respond for comment.