Tony Blair and John Major unite to warn there are just six days to save the UK

Appearing via video link at a Final Say rally in central London, Sir John closed his speech by saying: “So, with four words that, once upon a time, would have never easily passed my lips, I say tonight with great pleasure, Tony, over to you.”

Blair then walked on stage and announced: “Here are five words I never thought I would say – thank god for John Major.

“To John, to Michael Heseltine, for years I stood against you, tonight, it is an honour to stand with you.”

In an extraordinary intervention, Sir John endorsed three ex-Conservative independent candidates running against Boris Johnson’s party in the general election.

The Tory former prime minister said he would vote for ex-ministers David Gauke, Dominic Grieve and Anne Milton, who all lost the party whip earlier this year after rebelling on Brexit, if he lived in their constituencies.

He said: “Let me make one thing absolutely clear – none of them has left the Conservative Party, the Conservative Party has left them.

“Without such talent on its benches, Parliament will be the poorer, which is why, if I were resident in any one of their constituencies, they would have my vote.”

Meanwhile, Tony Blair said that while he personally would vote Labour, he urged others to back “the best candidates” in their constituencies.

He said: “There are great Labour candidates who deserve wholehearted support.

“I am voting Labour. But let me put it this way – if you look, constituency by constituency, you will know the best candidates to back. Back them.”

Sir John described Brexit as the “worst foreign policy decision in my lifetime”, and said leaving the EU will affect “nearly every single aspect of our lives for many decades to come”.

“It will make our country poorer and weaker. It will hurt most those who have least.

“Never have the stakes been higher, especially for the young. Brexit may even break up our historic United Kingdom,” he said.

Sir John urged voters to “choose the future you believe in”, telling young people in particular: “Your vote is absolutely crucial – for you have the longest lease on our country’s future, and our place in the wider world.

“Don’t wake up on Friday December 13 and regret not making a choice.”

Boris Johnson said that Sir John was “wrong” to have endorsed the candidates.

“I think it’s very sad and I think that he is wrong, and I think that he represents a view that is outdated, alas, greatly that I respect him and his record, and I think that what we need to do now is honour the will of the people and get Brexit done,” the prime minister told reporters.

He said: “This Conservative Party, which now expels the likes of Michael Heseltine, disowns the statesmanship of John Major, a party whose chancellor as of July this year is now exiled in the wilderness for the temerity to say what he knows to be true, namely that a no-deal Brexit is a risk no responsible Government would take.

“Such a Conservative Party does not deserve to govern unchecked and the country would not be wise to let them.

“It’s not Brexit that’s getting done. We’re getting done.

“This is the final chance for a final say. It’s not one general election but 650 individual ones. Think long. Think hard. Time to choose. Choose wisely.”

The two former prime ministers were joined by former Tory deputy prime minister Lord Heseltine and Blair’s former spin doctor Alastair Campbell.

Ex-Cabinet minister Gauke, who is standing as a candidate in South West Hertfordshire, was introduced at the rally on Friday evening alongside his father, who has featured in his campaign videos.

Former attorney general Grieve is hoping to be re-elected as the MP for Beaconsfield, while ex-minister Milton is seeking re-election in Guildford.

All three are standing as independents after losing the Tory whip in September when they backed a plan to take control of the Commons timetable to pass legislation to block a no-deal Brexit.