Brexit minister, Lord Frost has resigned “with immediate effect”, Press Association has reported.

This comes as he informed Boris Johnson that building a new relationship with the EU would be a “long-term task”.

Lord Frost has led negotiations with the EU but is now reported to have handed in his resignation to Boris Johnson last week with the agreement to leave in January.

In a letter to the Prime Minister released on Saturday evening, he said that he was “disappointed that this plan has become public this evening and in the circumstances, I think it is right for me to write to step down with immediate effect”.

Lord Frost thanked Mr Johnson and said, “Brexit is now secure”, but he said: “The challenge for the Government now is to deliver on the opportunities it gives us.

“You know my concerns about the current direction of travel.”

Lord Frost also said that he was sad the unlocking from Covid restrictions had not proved “irreversible” as promised and added: “I hope we can get back on track soon and not be tempted by the kind of coercive measures we have seen elsewhere.”

And he expressed his wish that the UK would become a “lightly regulated, low-tax” country.

In his reply, Mr Johnson he was “very sorry” to have received his resignation.

Lord Frost’s departure was described as a “watershed moment” by prominent Brexiteer Tory Andrew Bridgen.

Telling Time Radio, he said it was a “devastating blow for the Government and the Prime Minister” and suggested that many Conservative colleagues would be considering the PM’s future over Christmas.

In a tweet, he added Mr Johnson was “running out of time and out of friends to deliver on the promises and discipline of a true Conservative Government”.

He said: “Lord Frost has made it clear, 100 Conservative backbenchers have made it clear, but most importantly so did the people of North Shropshire.”

Lord Frost’s quitting piles more pressure on the PM, who has already suffered potentially his worst week politically since becoming Prime Minister with the rebellion, the loss of a former Tory safe seat in the North Shropshire by-election, and continued allegations over parties in Whitehall during lockdown restrictions.

In Northern Ireland, DUP leader Sir Jeffrey Donaldson said Lord Frost’s departure was a bad sign for Mr Johnson’s commitment to removing the Irish Sea border.

Sir Jeffrey said: “This government is distracted by internal strife, and Lord Frost was being frustrated on a number of fronts.

“We wish David well. We enjoyed a strong relationship with him and his team, but this raises more serious questions for the Prime Minister and his approach to the NI Protocol.”

Labour’s deputy leader Angela Rayner said the news showed “a Government in total chaos right when the country faces an uncertain few weeks”.

She tweeted: “@BorisJohnson isn’t up to the job. We deserve better than this buffoonery.”

Baroness Jenny Chapman, Lord Frost’s opposite number for Labour, added: “The Government is in chaos.

“The country needs leadership not a lame duck PM who has lost the faith of his MPs and Cabinet.

“Boris Johnson needs to get a grip, tell us his plan for the next few weeks and bring certainty for the people of Northern Ireland by unblocking the stalemate over the Protocol.”