John Major ​and Tony Blair have united to urge the British public to vote tactically in next week’s general election in order to boost the chances of securing a Final Say referendum on Brexit.

Calling on voters to put aside tribal loyalties in six days’ time, the former Labour and Conservative prime ministers warned voters against the prospect of a Boris Johnson majority government.

In an unprecedented intervention in the general election campaign, Sir John, who led the Tories in Downing Street for seven years, said the public should back rebel candidates running against Mr Johnson to block a hard Brexit.

Sir John insisted the prime minister’s decision to pursue “the worst foreign policy decision in my lifetime” risks breaking up the UK and making the country poorer and weaker.

He explicitly stated that he would vote for former ministers David Gauke, Dominic Grieve and Anne Milton – all running as independents after being thrown out of the party by Mr Johnson for their opposition to a hard Brexit – against their official Conservative rivals.

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Appearing via a video link at the pro-Remain rally, Sir John closed his remarks 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.”

Mr Blair, who defeated Sir John at the 1997 general election with a landslide win for the Labour Party, then walked onstage and told activists: “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.”

He told the Stop the Brexit Landslide event in central London that the 12 December election is “the last chance” for Britain’s electorate to keep open the possibility of a final public vote on the EU exit deal.

The two former PMs were joined by Sir John’s deputy prime minister Mr Heseltine and ex-justice secretary Mr Gauke in calling on voters to prevent a majority Johnson administration which would take the UK out of the EU without a confirmatory ballot on 31 January.

Describing Brexit as “the worst foreign policy decision in my lifetime”, Sir John went on: “It 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.”

Sir John hailed Mr Gauke, Mr Grieve and Ms Milton – standing as independents in the seats of South West Hertfordshire, Beaconsfield and Guildford which they formerly held as Tory MPs – as “principled, decent human beings” who should win the support of traditional Tory voters.

“Let me make one thing absolutely clear: none of them has left the Conservative Party, the Conservative Party has left them,” says the former Tory leader. “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.”

Tony Blair was also speaking at the event in London

Addressing young activists at the event, staged by the Vote for a Final Say and For Our Future’s Sake campaigns and supported by The Independent, Sir John added: “When the nation voted on Brexit they did so on a diet of fiction and undeliverable promises. As the facts become known, it is extraordinary that a new vote is denied: extraordinary, and undemocratic.

“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 13 December and regret not making a choice. So choose the future you believe in – for your own generation and the next.”

While saying that tribal loyalty has its place, he said: “No single party has a monopoly on wisdom ... Sometimes you need to vote with your head and your heart for your country and your future,” adding: “This is such a time.”

Mr Johnson said that his predecessor’s views were “outdated”. Asked about Sir John’s intervention at a campaign event in Kent, Mr Johnson said: “I think it’s very sad and I think he’s wrong.

Tony Blair addresses The Independent's Final Say rally demanding a second Brexit referendum (Clip)

“He represents a view that is outdated, alas. I respect him and his record. I must respectfully disagree with my illustrious predecessor and urge people on Thursday, ‘Come on, let’s get this thing done’.”

Mr Blair also called on voters not to give Mr Johnson a majority, saying: “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.”

Addressing the rally, The Independent’s editor, Christian Broughton, told those present: “There may be some people wishing they had shown up to vote 1,261 days ago in a certain referendum.

The Independent editor Christian Broughton addresses the Final Say rally demanding a second Brexit referendum (Clip)

“So much has changed since then. Don’t sit on the sidelines now: get out there on Thursday, get other people out, and do what you can for democracy, to win a Final Say for everyone.”

Vote for a Final Say announced that it has added Mr Gauke’s seat, along with foreign secretary Dominic Raab’s Esher and Walton and former Tory leader Iain Duncan Smith’s Chingford and Woodford Green, to the list of 25 key marginals where tactical voting by pro-referendum constituents can help prevent a Johnson majority.

The organisation is spending thousands of pounds on targeted advertising in the seats, urging voters to back the former justice secretary, Mr Raab’s Lib Dem rival Monica Harding and Mr Duncan Smith’s Labour opponent Faiza Shaheen.