Former prime minister Tony Blair claims the Brexit "nightmare" will not be over if the Tories win the election and take the UK out of the EU on 31 January.
Visiting north east Wales, Mr Blair said people would be "shocked and angry" at how tough negotiations on the future trading relationship would be.
"I wish it were as simple as 'just do it', but it isn't," he said of Boris Johnson's vow to "get Brexit done".
"Anybody who's telling you that it is that simple is conning you."
Speaking to BBC Wales, Mr Blair said: "I think the British people are being told… that if you vote Conservative on the 12th of December then the Brexit nightmare ends, and it obviously doesn't.
"Anybody who's studying the detail of this knows that the moment you leave on the 31st January you then go into the next Brexit negotiation which, by the way, is going to be much tougher than the last one.
"And I think when people find this out they are going to be shocked and angry, and this is reality."
Mr Blair, who was Labour's longest-serving prime minister, stressed he would be backing the party in the general election despite his differences with current leader Jeremy Corbyn.
He said: "The question people have to ask themselves is: do you want a Conservative Party led by Boris Johnson - who in my view is unreliable - ending up able to do what they want on Brexit, including no-deal.
"I still believe in the values of the Labour Party," Mr Blair added.
"But it is a strange time in politics and I won't say I haven't struggled over the direction the Labour Party has taken because I have, and that's obvious."