Two former prime ministers – Sir John Major and Tony Blair – are to address a Final Say rally where supporters of a second EU referendum will call for tactical voting to block a Conservative majority government under Boris Johnson.

The rally in London, six days ahead of the 12 December general election, is the most explosive intervention in the campaign so far by the two ex-PMs, both of whom are staunch opponents of Brexit.

It comes as the Vote for a Final Say campaign announced it is throwing resources at the seats of Dominic Raab, the foreign secretary, and former Tory leader Iain Duncan Smith in the hope of delivering the tactical votes which could oust the two prominent Brexiteers.

No details are yet available of the content of the video message to be given by former Tory PM Major, or whether he will explicitly call on voters to vote against the Conservatives.

But his message will be heard alongside speeches from his former deputy prime minister Michael Heseltine, who has already said he would vote Liberal Democrat to stop Brexit, and ex-justice secretary David Gauke, who is standing as an Independent in next week’s ballot and will accuse Mr Johnson of running a “fundamentally dishonest” campaign.

Best pictures from Final Say Brexit march Show all 76 1 /76 Best pictures from Final Say Brexit march Best pictures from Final Say Brexit march Crowds march through central London Getty Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march Protesters gather in Parliament Square Led By Donkeys Best pictures from Final Say Brexit march Mayor of London Sadiq Khan joins protesters PA Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march Anti-Brexit protesters fill Parliament Square in London PA Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march A demonstrator marches EPA Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march Protesters push a float depicting Dominic Cummings using Boris Johnson as a puppet during the Final Say Brexit march in London PA Best pictures from Final Say Brexit march EU supporters react after the result of the vote on the deal delay was announced at the House of Commons REUTERS Best pictures from Final Say Brexit march A demonstrator carries his dog draped in EU flag AP Best pictures from Final Say Brexit march Thousands of people taking part in a People's Vote march AFP/Getty Best pictures from Final Say Brexit march Protesters use their flags to shelter from the rain Getty Images Best pictures from Final Say Brexit march Protesters march towards Parliament Square Getty Images Best pictures from Final Say Brexit march Anti-Brexit demonstrators carry placards and EU flags AP Best pictures from Final Say Brexit march Sir Oliver Letwin MP in Parliament Square, London, during an an anti-Brexit, Let Us Be Heard rally, after it was announced that the Letwin amendment, which seeks to avoid a no-deal Brexit on October 31, has been accepted PA Best pictures from Final Say Brexit march Thousands of people taking part in a People's Vote march UK BROADCASTERS POOL/AFP via Get Best pictures from Final Say Brexit march A protester with "Bollocks to Brexit" stickers on his head Getty Images Best pictures from Final Say Brexit march Demonstrators march EPA Best pictures from Final Say Brexit march Britain's main opposition Labour Party shadow Secretary of State for Exiting the EU Keir Starmer, shadow Home Secretary Dianne Abbott, shadow Chancellor of the Exchequer John McDonnell, shadow Foreign Secretary Emily Thornberry speaks on stage in Parliament Square AFP via Getty Images Best pictures from Final Say Brexit march Anti-Brexit supporters cheer outside parliament AP Best pictures from Final Say Brexit march A EU supporter waves flags REUTERS Best pictures from Final Say Brexit march Anti-Brexit supporters AP Best pictures from Final Say Brexit march Protesters shout and chant demanding a final say Getty Images Best pictures from Final Say Brexit march Police look towards protesters Getty Images Best pictures from Final Say Brexit march Pro-Brexit protesters outside the Houses of Parliament in London PA Best pictures from Final Say Brexit march People taking part in an Anti-Brexit, Let Us Be Heard march head to Parliament Square in London PA Best pictures from Final Say Brexit march People taking part in an Anti-Brexit, Let Us Be Heard march in Trafalgar Square PA Best pictures from Final Say Brexit march Anti-Brexit supporters cheer outside parliament AP Best pictures from Final Say Brexit march People taking part in an Anti-Brexit, Let Us Be Heard march past Trafalgar Square PA Best pictures from Final Say Brexit march Protesters shout and chant Getty Images Best pictures from Final Say Brexit march Cory (7) sits on his father's shoulders as protesters march towards Parliament Square Getty Best pictures from Final Say Brexit march People attend the 'Together for the Final Say' march EPA Best pictures from Final Say Brexit march Protesters march towards Parliament Square Getty Images Best pictures from Final Say Brexit march Mayor of London Sadiq Khan speaks on stage PA Best pictures from Final Say Brexit march A woman in EU costume EPA Best pictures from Final Say Brexit march Anti-Brexit protesters fill Parliament Square in London PA Best pictures from Final Say Brexit march Anti-Brexit protesters AP Best pictures from Final Say Brexit march Anti-Brexit protesters hold an Independent banner Angela Christofilou/The Independent Best pictures from Final Say Brexit march Protesters hold an anti-Brexit placard by The Independent stall during the Final Say Brexit march in London Tom Richell Best pictures from Final Say Brexit march Anti-Brexit protesters in Parliament Square, London, PA Best pictures from Final Say Brexit march Demonstrators hold placards and EU flags AFP/Getty Best pictures from Final Say Brexit march An anti-Brexit protester joins in the Final Say Brexit march on 19 October AP Best pictures from Final Say Brexit march EU supporters march in London REUTERS Best pictures from Final Say Brexit march Sir Patrick Stewart and Paul McGann join in the Final Say Brexit march in London on 19 October PA Best pictures from Final Say Brexit march EU supporters march Reuters Best pictures from Final Say Brexit march A demonstrator shouts into a megaphone as she marches AFP via Getty Images Best pictures from Final Say Brexit march A dog dressed in an anti-Brexit placard is walked in the Final Say Brexit march in London EPA Best pictures from Final Say Brexit march EU supporters march REUTERS Best pictures from Final Say Brexit march A morris dancer takes part in the Final Say Brexit march in London PA Best pictures from Final Say Brexit march Whippets are dressed in EU flag clothing during the Final Say Brexit march in London AP Best pictures from Final Say Brexit march Protesters march towards parliament in the Final Say Brexit march in London on 19 October 2019 PA Best pictures from Final Say Brexit march An anti-Brexit demonstrator takes part in the Final Say Brexit march in London AFP/Getty Best pictures from Final Say Brexit march EU supporters call on the government to give Britons a vote on the final Brexit deal Reuters Best pictures from Final Say Brexit march Anti-Brexit protesters Angela Christofilou/The Independent Best pictures from Final Say Brexit march An anti-Brexit protester Angela Christofilou/The Independent Best pictures from Final Say Brexit march Anti-Brexit protesters demonstrate in London during the Final Say Brexit march on 19 October AP Best pictures from Final Say Brexit march Protesters push a float depicting Dominic Cummings using Boris Johnson as a puppet during the Final Say Brexit march in London AFP/Getty Best pictures from Final Say Brexit march Protesters march towards parliament from Park Lane in the Final Say Brexit march in London on 19 October 2019 PA Best pictures from Final Say Brexit march Marchers visit The Independent stall during the Final Say March in London Tom Richell Best pictures from Final Say Brexit march An anti-Brexit protester holds a sign calling for a Final Say during a march in London EPA

“This general election is about Brexit,” Mr Gauke will say. “And for my former party it is about one soundbite – ‘Get Brexit Done’.

“It is an offer to the country that is fundamentally dishonest. If we follow Boris Johnson’s plan, then we are guaranteed to be dealing with Brexit for years ahead. Rather than putting it behind us, we’ll be plunged into a negotiation that looks doomed to see us leave on WTO terms at the end of next year and yet more years of uncertainty.

“I worked to find a compromise that sought to mitigate the worst effects of Brexit but the hardliners in my former party wouldn’t accept anything other than the hardest of hard Brexit.

“Now the choices are very stark. The country is polarised. It is either a hard Brexit or Remain. Neither option has a mandate from the 2016 referendum. I believe we need to put those options back to the British people in a confirmatory referendum. And in those circumstances, it is overwhelmingly in the national interest to Remain.”

Vote for a Final Say is spending thousands of pounds on targeted advertising in each of 25 key marginal seats where just a few hundred people could yet help halt a Johnson majority.

Last night, after additional funds were secured, the campaign announced it would extend its activities to three more constituencies currently held by the Conservative Party, including South West Hertfordshire, where Mr Gauke – who was expelled by Mr Johnson for voting against a no-deal Brexit – is fighting to prevent Conservatives regaining his seat. The campaign is also backing Lib Dem Monica Harding’s drive to unseat Mr Raab in Esher and Walton and Labour candidate Faiza Shaheen’s campaign against Mr Duncan Smith in Chingford and Woodford Green.

Mr Blair’s former director of communications Alastair Campbell said the rally will send the message that voters who want to stop Mr Johnson gaining total power and imposing a hard Brexit will have to “hold their noses” and vote for whatever party has the best chance of beating Tories in their constituency.

“I have spent the past few days touring marginal constituencies,” said Mr Campbell.

Tony Blair: ‘This is why Labour must not agree to this election ploy until the situation is dealt with’ (PA)

“In some, I have been urging Labour supporters to back the Liberal Democrats. In others, I am asking Liberal Democrats to vote Labour. It may be the only way to stop Boris Johnson from winning big and taking this country well to the right.

“We have seen already how dangerous he is with just a little power. Imagine him with a lot.

“In an ideal world, we would all just vote for the party or candidate who best represents our views and values. But in so many seats, voting Lib Dem or Green against Labour helps only the Tories, and in others voting Labour against Lib Dems helps the Tories too.

“This election is not about Jeremy Corbyn or Jo Swinson – neither can win a majority. It has come down to one simple question: Do you want to give Boris Johnson and the reckless right-wing gang that surrounds him, total power to do whatever they want?

“It is not too late to stop him and secure a democratic Final Say on Brexit. But a lot more people are going to have to focus on who can win their seat, hold their noses, and put aside party loyalty or even views about the respective leaders if we are to have a real chance.”

The rally, organised jointly by Vote for a Final Say and the anti-Brexit youth group For our Future’s Sake, is backed by The Independent, whose editor Christian Broughton is among the speakers.

Also speaking will be Tim Walker, who quit as the Liberal Democrat candidate in Canterbury to encourage his supporters to help pro-referendum Labour MP Rosie Duffield fend off a challenge from a Brexit-backing Conservative.

Vote for a Final Say is spending thousands of pounds on targeted advertising in each of 25 key marginal seats where just a few hundred people could yet help halt a Johnson majority.

Last night, after additional funds were secured, the campaign announced it would extend its activities to three more constituencies currently held by the Conservative Party.

Isi Daley, a young activist with FFS, said that tactical votes to prevent a Johnson majority would be a vote for the future of younger generations.

“This election is different,” said Ms Daley. “It will determine the future of this country – my future – for decades. Many people have realised that Brexit, in any form, is bad for the country. But with Boris Johnson in power, Brexit wouldn’t just be bad, but dangerous.