Nick Clegg has accused the Tories of being stuck "up the Brexit creek" without a paddle, canoe or map.
Making his return to centre stage at the Liberal Democrat conference, the former deputy prime minister delivered a stern warning that "swivel-eyed" Tory backbenchers will force Britain out of the single market, with dire consequences for the UK economy.
"You cannot have untrammelled access to a single market, which remember is a single market of rules, without abiding in one shape or form by those rules. That is what will lead to gridlock in the next few years.
"That is why they find themselves up this Brexit creek, never mind that they don't have a paddle, they don't have a canoe, they don't have a map, they have absolutely no clue whatsoever," he said.
Mr Clegg, who has returned to the political front line as Lib Dem Brexit spokesman, insisted it was essential for British prosperity to remain in the single market, which allows free movement of trade and people.
He warned that the two sides of the Tory brain were being torn between "free trade and gunboat diplomacy".
"I think, under pressure from their swivel-eyed backbenchers, under pressure from the, sort of, Brexit press, under pressure from their own internal contradictions, they will move remorselessly towards a hard Brexit.
"Not only taking us out of the European Union, but taking us out of the single market as well. And when they do that they will do untold damage to the British economy," Mr Clegg said.
The intervention came as Lib Dem leader Tim Farron accused Jeremy Corbyn of "throwing in the towel" over EU membership.
Attacking the Labour leader, Mr Farron said: "He didn't make an effort to keep us in Europe during the referendum campaign and now he has made clear he won't fight for our membership of the single market."
Mr Farron said he felt a sense of "bereavement" at the EU referendum result.
"There is a sense of bereavement because it wasn't just about losing an election, there was a sense of losing your identity.
"Whilst we've not changed our view that the referendum result is one we should respect, neither have we changed our view that being in Europe is undoubtedly still the best thing for the Liberal Democrats," Mr Farron told BBC Radio 5 Live.
The Lib Dem conference in Brighton voted to back Mr Farron's demand for the terms of a Brexit deal to be put to a second referendum.