Lord Frost has revealed he quit as Brexit minister because he cannot support “coercive” policies on COVID.

Speaking for the first time since his resignation, Lord Frost said he had “huge admiration” for Boris Johnson and insisted they never disagreed “in any way about Brexit policy right up to the last day”.

The former lead Brexit negotiator, who quit on Sunday, said: “I left the government, as I think is well known, because I couldn’t support certain policies, most recently on COVID restrictions and ‘plan B’.

“If you’re a minister, you have to support collective responsibility, you have to support decisions of the government, and I couldn’t so that’s why I had to leave.”

Analysis: Lord Frost’s resignation marks a chaotic end to possibly Boris Johnson’s worst week as PM

Lord Frost said he was “absolutely confident this country has a great future under Boris Johnson’s leadership if we can get the policies right”.

“I’m absolutely confident he’s the right man to take the country forward,” he added.

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“I don’t support coercive policies on COVID. The prime minister’s got some very difficult decisions to take and I’m sure he’ll be thinking very hard on them.”

He added that his departure is “absolutely not about leadership, this is about policy differences”.

Lord Frost revealed he intended to leave his role in the New Year but it had been announced “earlier than we thought”.

Liz Truss will take on Lord Frost’s Brexit responsibilities while keeping her roles as foreign secretary and minister for women and equalities.

Lord Frost added that he is sure Ms Truss and MP Chris Heaton-Harris, who has been made Europe minister, “are going to do a great job”.

Ms Truss tweeted that she was “pleased” to be taking over.

The Northern Ireland Protocol is the main sticking point in negotiations and is designed to avoid the introduction of a hard border between Ireland and Northern Ireland.

Responding to Ms Truss’ appointment, Maros Sefcovic, the European Commission’s vice president, said: “My team and I will continue to cooperate with the UK in the same constructive spirit on all important tasks ahead, including the Protocol on Ireland/Northern Ireland.”

Lord Frost’s resignation is seen as a severe blow for Boris Johnson, following a week in which almost 100 Conservative MPs voted against vaccine passports and the Tories lost the North Shropshire by-election.

Sky’s senior Ireland correspondent, David Blevins, said Lord Frost’s departure “couldn’t come at a more sensitive moment as negotiations on the Northern Ireland Protocol near their culmination”.

Northern Ireland’s former first minister, Arlene Foster, described Lord Frost’s resignation as “enormous”.