Brexit negotiations will "founder" if Thursday's General Election results in a coalition government led by Jeremy Corbyn, Foreign Secretary Boris Johnson has warned.
Mr Johnson said the "herbivorous" Labour leader would be "eaten for breakfast" by Brussels bureaucrats.
He warned that a Corbyn administration propped up by the Scottish National Party and Liberal Democrats would give in to the "jabbering" of Nicola Sturgeon and Tim Farron who want Britain to stay in the EU.
In a speech in County Durham, Mr Johnson said he "shuddered" at the prospect of the Labour leader taking the helm of Brexit talks.
"It makes me shudder to think that we could seriously be about to elect a Corbyn-led administration that would impose destructive new taxes on businesses, on homes, on gardens - at the very moment when we should be about to go forward with a great Global Britain," he said.
"This is the moment to believe in the huge potential of Brexit Britain.
"It is absolutely vital on Thursday that we elect a government that will negotiate with courage, determination, optimism and belief in the outcome.
"Let's get Brexit done. Let's get Brexit right. Let's believe in Britain. And let's work for the next 48 hours to make sure our negotiations are led by Theresa May and the Conservatives."
Mr Johnson said only the Conservatives were "willing to stick up for the democratic will of the British people" and take advantage of the "moment of hope and opportunity" represented by Brexit.
He warned: "For 30 years, (Corbyn) has been soft and muddle-headed on terror. He has been soft and muddle-headed on defence. He has taken the side of just about every adversary this country has had in my lifetime, from the IRA to Hamas, from Soviet communism to General Galtieri.
"I don't mean to compare our European friends to any of these people, but it is psychologically impossible to imagine him having the grip or the firmness to get the right Brexit deal for this country.
"A Labour negotiating team would arrive in Brussels like a family of herbivores at a watering hole of lions. They would be eaten for breakfast."
Mr Johnson said that there were "plenty of people in Brussels and some in this country who have spent night and day since June 23 dreaming, hoping, scheming and plotting that they will be able to reverse that decision".
He asked: "Do we want them to frustrate the will of the people? Do we want them to overturn our democratic decision of last year? Do we want them to frustrate Brexit? I don't think we do."
Mr Corbyn lacked the "firmness of purpose" to take on the "glutinous conglomerate of privileged interests" that made up the EU establishment, said the Foreign Secretary.
"Just imagine the scene in Brussels if Jeremy Corbyn were to mosey in to take his seat opposite Jean-Claude Juncker, Angela Merkel, (Emmanuel) Macron, (Michel) Barnier and all the rest of them," said Mr Johnson.
"I think they would look at him with what is called 'a wild surmise'. Not just because they don't know what his position is, but because he doesn't know what he really wants."
The likely reliance of any Labour administration on SNP and Lib Dem votes for its Commons majority would leave Mr Corbyn like the two-headed Zaphod Beeblebrox character from The Hitchhiker's Guide To The Galaxy, Mr Johnson said, "with Nicola Sturgeon jabbering in one ear and Tim Farron in the other, both of them telling him to do exactly what Brussels wants, because both of them are 100% committed to reversing that decision of June 23".
"How on Earth would Corbyn be able to construct a logical negotiating position with that pair?" the Foreign Secretary asked.
Mr Johnson added that younger voters who did not remember the era of nationalisation, Soviet communism and socialism did not necessarily realise the consequences of Mr Corbyn's policies.
"I do," he said. "We don't want it back in this country. It would be an absolute disaster ... I've never seen such a left-wing leader with such a left-wing approach to politics.
"It would seriously undermine the credibility of this country in international negotiations, and it would be a disaster for the delivery of our our Brexit talks.
"In my view, it would lead to the frustration of the democratically expressed will of the British people last year."