Boris Johnson has said it is "not too late to save Brexit" as he accused Theresa May of "dithering" over the UK's strategy for leaving the EU.
In a resignation speech in the Commons, the former foreign secretary said a "needless fog of self-doubt" had descended over the past 18 months.
While praising the PM's "courage and resilience", he said her Chequers plan would see the UK in "miserable limbo".
He quit 10 days ago, saying he could not support the PM's EU blueprint.
BBC chief political correspondent Vicki Young said Mr Johnson's friends were insisting he was not trying to emulate Sir Geoffrey Howe, whose caustic resignation speech in 1990 is widely seen as having paved the way for Margaret Thatcher's downfall weeks later.
But by suggesting there was time for Mrs May to change her approach, she said he appeared to be telling MPs that it would be "in their hands" if she didn't.
Addressing MPs, Mr Johnson said the "bright certainties" that followed the 2016 Brexit vote had dissipated and the UK risked ending up in "economic vassalage" if it agreed to follow EU regulations on trade, the environment and social affairs.
He said the vision set out by Mrs May in her Lancaster House speech in January 2017 for a "strong independent, self-governing Britain" had never been turned into a firm negotiating position.
Joke-free and pretty savage
By Laura Kuenssberg, the BBC's political editor
It's the first Boris Johnson speech that I can remember watching that didn't have any jokes. For his allies, that was the point. It wasn't designed, they say, to make a personal attack on Theresa May.
But my goodness, it made a pretty savage attack on her policies - a "fog of self-doubt", "pretending to the electorate", "a fantastical Heath Robinson" proposal on customs, "miserable permanent limbo".
For someone who less than a fortnight ago celebrated collective cabinet responsibility at Chequers, that's quite some feat.
The government, he claimed, had simply not tried to make the case for a loose arrangement with the EU based on a wide-ranging free trade deal.
Instead, he said the UK had conceded ground over the divorce bill, the role of the European Court of Justice and the Northern Ireland border, which he said had needlessly become "politically charged" and resulted in a "fantastical, Heath Robinson" customs arrangement being put forward.
Ministers, he warned, were "saying one thing to the EU about what we are really doing, and pretending another to the electorate" - a situation which voters would see through.
Read my personal statement to the House of Commons this afternoon. pic.twitter.com/Zy6klFWQsr— Boris Johnson (@BorisJohnson) July 18, 2018
"Given that in important ways this is Brexit in name only, I am of course unable to support it, as I said in cabinet at Chequers, and am happy to be able now to speak out against it.
"It is not too late to save Brexit. We have time in these negotiations. We have changed tack once and we can change again.
"We need to take one decision now before all others - and that is to believe in this country and what it can do because the UK's admirers across the world are fully expecting us to take back control."
Mr Johnson's speech was retweeted by a host of prominent Brexiteer Tories, among them Jacob Rees-Mogg, chairman of the influential European Research Group of MPs.
The speech of a statesman. https://t.co/2H98U9hJJJ— Jacob Rees-Mogg (@Jacob_Rees_Mogg) July 18, 2018
At the same time as Mr Johnson was attacking her Brexit strategy, Mrs May was defending it before the Liaison Committee of MPs.
She conceded that her proposed post-Brexit trade and tariff arrangements were a "novel idea" that did not exist anywhere else in the world, adding that that did not matter, if they were the "best" arrangements for the UK.
Earlier she had said at PMQs that the final Brexit deal must honour the 2016 referendum result but also be "workable" in terms of protecting jobs and livelihoods.
On Wednesday evening Mrs May addressed the 1922 Committee of backbench Tory MPs, where one - Simon Clarke - announced he had withdrawn a letter calling for a vote of no confidence in the PM.
Mr Clarke said he remained "deeply opposed" to the Chequers agreement but that Mrs May deserved a chance to negotiate and "see what comes back".
Tempers have been running high in the Commons over planning for the UK's March 2019 exit.
In the past 48 hours, the government has narrowly won a series of votes on trade and customs arrangements despite substantial rebellions by pro-European Tories.
Would-be rebels were reportedly warned on Tuesday that opposing the government in a vote on a customs union would lead to a vote of confidence and potentially a general election.
The government ultimately won the vote by a margin of six.