British Brexit minister David Frost resigned on Saturday over disillusionment with the direction of Boris Johnson's government, dealing a major blow to the embattled prime minister as the Omicron variant sweeps the country.

The resignation of Frost, a core architect of Johnson's tumultuous Brexit strategy, raised questions about the future tone of the EU divorce and the immediate course of talks on Northern Ireland. It also added to a sense of turmoil in Johnson's Conservative government.

Downing Street released Frost's resignation letter, in which he said he would step down with immediate effect as "Brexit is now secure." But he said he had made clear to Johnson his "concerns about the current direction of travel."

"I hope we will move as fast as possible to where we need to get to: a lightly regulated, low-tax, entrepreneurial economy, at the cutting edge of modern science and economic change," he added.

"It is disappointing 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," he said in the letter.

His resignation was first reported by The Mail on Sunday, which said it was triggered by Johnson's tougher COVID restrictions but also by a broader discontent with tax rises and the cost of environmental policies.

Johnson said he was sorry to receive Frost's resignation.

The departure of the British government's most senior Brexit negotiator comes on top of warnings from some of his Johnson's own Conservative Party lawmakers that he must improve his leadership or face a challenge.

Johnson, who won a landslide election victory in December 2019, is facing the biggest crisis of his premiership after a litany of scandals and missteps, which his opponents say show he is unfit to be prime minister.

He has faced a barrage of criticism since a video emerged showing his staff laughing and joking about a Downing Street party during a 2020 Christmas lockdown when such festivities were banned.

Downing Street had denied a party took place. Britain's top civil servant, Simon Case, has stepped down from leading the investigation into alleged parties after it was disclosed that an event had been held in his own office.

The loss of a parliamentary seat in an election defeat in a Conservative stronghold earlier this week showed public dismay over the litany of scandals and stepped up pressure on Johnson.

(With input from Reuters, AFP)