Theresa May has faced calls to grant a second referendum on Brexit as a former Tory Cabinet minister and the leader of the Liberal Democrats joined forces on the issue.
At a rally in Manchester to mark the start of the Conservative Party conference, former health secretary Stephen Dorrell and Lib Dem leader Sir Vince Cable demanded a vote on the final Brexit deal.
European Movement chairman Mr Dorrell told the rally that people should be given the "opportunity to change their minds".
Quoting his former boss Sir John Major, Mr Dorrell said Brexit would be a "historic mistake".
He said: "Many people say to me I'm flying in the face of democracy. What is undemocratic about changing your mind?"
Sir Vince said there was a "civil war" going on at the Tory conference between the Prime Minister and Foreign Secretary Boris Johnson.
"They are fundamentally divided, disunited, disorganised and this is the government that is supposed to be negotiating for Britain," he said.
"It is a mess and, unfortunately, the negotiations are going to lead to a mess."
Broadcaster Terry Christian hit out at "spineless" Greater Manchester mayor Andy Burnham for refusing to oppose the Brexit process.
The former host of The Word said "the jellyfish known as Andy Burnham" was part of a "cowardly Labour Party".
The rally - which featured protest songs led by a lookalike of the Foreign Secretary riding an inflatable unicorn - was part of a protest by thousands of Remain supporters, many waving EU flags.
The gathering in the shadow of Manchester Cathedral followed a march through the city centre.