British Brexit minister David Frost resigned on Saturday (December 18) over disillusionment with the direction of Boris Johnson's government,
It deals a major blow to the embattled prime minister.
The resignation of Frost, an architect of Johnson's Brexit strategy, raised questions about the future tone of the EU divorce.
Frost said he was confident that Brexit was secure, but said he had concerns about the government's direction.
"You know my concerns about the current direction of travel," Frost told Johnson in a letter released by Downing Street.
He added that Britain needs "a lightly regulated, low tax, entrepreneurial economy."
His resignation was first reported by the Mail on Sunday, which said it was triggered by Johnson's tougher COVID restrictions and a broader discontent with tax rises and the cost of environmental policies.
Frost said he'd agreed with Johnson earlier this month to leave in January, but because his move had been leaked it should happen with immediate effect.
"We also need to learn to live with COVID," Frost said. "I hope we can get back on track soon and not be tempted by the kind of coercive measures we've seen elsewhere."
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's under pressure over a series of scandals, including a video which emerged showing his staff laughing and joking about a Downing Street party during a 2020 Christmas lockdown when such festivities were banned.