Transport minister Jo Johnson has quit the government, calling for the public to have a fresh say on Brexit.
The MP, who is Boris Johnson’s brother, said the withdrawal deal currently being negotiated with the European Union “will be a terrible mistake”.
Arguing Britain was “on the brink of the greatest crisis” since World War Two, he said what was on offer wasn’t “anything like what was promised”.
Downing Street thanked him for his work but ruled out another referendum.
Jo Johnson voted to remain in the EU while his brother Boris, who quit as foreign secretary in July, was a leading Brexiteer.
His brother praised his decision, saying they were “united in dismay” at the PM’s handling of the negotiations.
Cabinet ministers have been invited this week to read the UK’s draft withdrawal deal with the EU. Theresa May has said the withdrawal deal is 95% done – but there is no agreement yet on how to guarantee no hard border in Northern Ireland.
In an article sent to journalists, the MP for Orpington said he would vote against the withdrawal agreement in the Commons.
“Hopes for the ‘easiest trade deal in history’ have proved to be delusions,” he wrote.
“Contrary to promises, there is in fact no deal at all on our future trading relationship with the EU which the government can present to the country,” he wrote.
“While we wait to negotiate trading terms, the rules of the game will be set solely by the EU. Britain will lose its seat at the table and its ability to amend or vote down rules it opposes.”
The BBC’s political editor Laura Kuenssberg
For some time, Jo Johnson has struggled with the unfolding reality of Brexit.
A well-respected and liked member of the government, he has decided that what was promised to people during the referendum campaign is now so different to what is on the table that he has decided to quit the government instead.
He’s not the first, nor the best-known minister to resign over Brexit. But to leave at this moment, right when Theresa May is trying to stitch together a final deal, could have a serious impact.
He added that, given Brexit had “turned out to be so far from what was once promised, the democratic thing to do is to give the public the final say”.
“This would not be about re-running the 2016 referendum, but about asking people whether they want to go ahead with Brexit now that we know the deal that is actually available to us, whether we should leave without any deal at all or whether people on balance would rather stick with the deal we already have inside the European Union.”
He added: “Britain stands on the brink of the greatest crisis since the Second World War. My loyalty to the party is undimmed. I have never rebelled on any issue before now. But my duty to my constituents and our great nation has forced me to act.”
In response, a Downing Street spokesman said: “The referendum in 2016 was the biggest democratic exercise in this country’s history. We will not under any circumstances have a second referendum.
“The prime minister thanks Jo Johnson for his work in government.”
Mr Johnson is the sixth minister in Theresa May’s government to resign specifically over Brexit, following David Davis, Boris Johnson, Philip Lee, Steve Baker and Guto Bebb.
For Labour, Shadow Brexit Minister Jenny Chapman said Mrs May had “lost all authority and is incapable of negotiating a Brexit deal within her own party, let alone with the EU”. Tom Watson, Labour’s deputy leader, added: “How many more resignations is it going to take before we see a change at the top?”
Brexiteer Tory MP Andrea Jenkyns tweeted that she did not agree with him about another referendum – but his intervention highlighted unease on both sides of the debate, with the PM’s efforts to secure a deal.
And pro-Remain Conservative Anna Soubry supported his decision and said it was time for another referendum.
David Davis, who quit as Brexit Secretary over Mrs May’s Chequers Brexit plan, tweeted: