The former deputy prime minister accused Theresa May and Jeremy Corbyn of "colluding" to form a "pact of silence" to avoid discussing EU withdrawal during the General Election.
The Liberal Democrat politician added Mrs May will lack the ability to strike "excruciatingly difficult, controversial" compromises with Brussels as she will lead a "merged" Tory-Ukip party if returned as Prime Minister.
He also sought to warn about the impact of Brexit on prices, immigration, security and the NHS.
Asked about the chances of the Brexit talks collapsing, Mr Clegg said in the immediate aftermath of the referendum it did not cross his mind it could occur as it is an "extraordinarily destructive" thing for both the UK and EU.
He told a campaign event in east London: "So it's infinitely dispiriting to hear later the chances of it collapsing altogether are much, much higher than I could have ever possibly imagined.
"That's only been confirmed during the course of the General Election campaign.
"A proper leader of a country would use this election campaign not just to secure a mandate on Thursday, a majority, but also to explain and prepare the British public for the compromises, the huge, excruciatingly difficult, controversial, sometimes wholly unpopular compromises, that are inevitable if you want a deal.
"And yet Theresa May has made no attempt whatsoever to prepare people for what's to come.
"It's a spectacular failure of leadership.
"She's digesting close to four million Ukip voters in one sitting, she in effect only does what the editor of the Daily Mail tells her to do these days as far as I can make out, so she has no room for manoeuvre because she will preside over, in effect, a new party - a merged party between the Conservative Party and Ukip."
Mr Clegg pointed to falls in the pound's value against the euro and US dollar since June 2015 and average earnings failing to increase at the same rate as prices.
He added: "Consumers are already beginning to feel the Brexit squeeze.
"Price rises have hit energy bills, petrol, and clothes.
"It's enough to make anyone need a fortifying glass of wine - but last week, it was reported that the average price of a bottle of wine has hit its highest price ever.
"And as thousands of families prepare for their summer holidays abroad, the devaluation of the pound will hit them in the pocket, too.
"Everything paid for in euros, everything paid for in dollars, from accommodation to ice cream, will be more expensive."