David Davis, who resigned on Sunday as UK Brexit minister, claimed in a stinging broadside on Monday that the government was giving away too much, too easily to the European Union.
Mr Davis said he had decided to quit because he did not believe in the "dangerous" strategy thrashed out at a stormy cabinet meeting last week and believed that he was no longer the best person to lead negotiations.
His exit raises questions about the position of Prime Minister Theresa May, with further departures following months of infighting within her leadership circle possible.
Mrs May appointed junior minister Dominic Raab, a former diplomat who was a prominent campaigner for Leave in the 2016 referendum, as Mr Davis's successor.
Mr Raab has the job of persuading Brussels to accept Britain's offer of close economic ties following the UK's exit in March next year. Negotiations are expected to conclude in October before the UK leaves the world's largest trading bloc.
Focus is likely to switch to Foreign Secretary Boris Johnson, who has sniped at the premier's policy and is a significant figure within a faction that wants a clean break from the European Union.
"It seems to me, we're giving too much away and too easily," Mr Davis told the BBC. "And that's a dangerous strategy at this time."
Mr Davis said that his departure was not a prelude to him challenging the prime minister’s position, but said it was “not for me to make other peoples’ decisions” on whether to follow his lead and to leave the cabinet. Mr Davis’s deputy, Steve Baker, also resigned.
The decision by Mr Davis swiftly ended the impression that Mrs May had pulled off a political masterstroke by pushing through her vision of Brexit while keeping rival factions of her party onside.
Ministers gathered for a retreat at the premier's country residence, Chequers, on Friday for talks called at short notice, and mobile phones had to be left outside. It concluded with an announcement of a strategy that sought to allay some concerns in Brussels and within the business community over trading links.
However critics, including Mr Davis, felt that it meant that the plan would tie the UK too closely to EU decision-making in the future, and would give the UK parliament only “illusory” powers.
Mr Davis had disagreed with the PM’s plans for keeping EU rules for goods and adopting a close customs partnership with the other 27 member countries.
As the minister responsible for the Brexit negotiations, Mr Davis was a key voice in the debate in the UK.
"I had to do something I didn't believe in and wouldn't work as well as other strategies," said Mr Davis. In his letter to the prime minister explaining his decision, Mr Davis said he felt it was "less and less likely" that Britain would leave the customs union and the single market, articles of faith for Brexiteers.
“The general direction of policy will leave us in at best a weak negotiating position and possibly an inescapable one,” he said in his letter.
He said that he felt he had to resign as he had no desire to be a “reluctant conscript” in the selling the prime minister’s deal in negotiations with the EU.
In her response, Mrs May said that “I do not agree with your characterisation of the policy we agreed at cabinet on Friday. Parliament will decide whether or not to back the deal the government negotiates, but that deal will undoubtedly mean the returning of powers from Brussels to the United Kingdom. The direct effect of EU law will end when we leave the EU.
“Choosing not to sign up to certain rules would lead to consequences for market access, security co-operation or the frictionless border, but that decision will rest with our sovereign parliament, which will have a lock on whether to incorporate those rules into the UK legal order.”
Mrs May will have a key meeting with members of her Tory party to discuss her plan in parliament on Monday. Some politicians have already expressed their misgivings.
Jacob Rees-Mogg, a key Eurosceptic figure in the ruling Conservative party, said Mr Davis’s resignation was “very important that he has given great reassurance to backbench MPs” who opposed the Chequers agreement.
Asked if the prime minister could survive. Mr Rees-Mogg refused to support her and said that “she would be well advised to revisit her Brexit policy. It’s more important to do what you have promised the British electorate than stick by lines in the sand.”
Mrs May came under fire from Jeremy Corbyn, leader of the opposition Labour Party, who said she "has no authority left and is incapable of delivering Brexit".