Boris Johnson has urged Jeremy Corbyn to drop his plan to “fiddle” a second Brexit referendum by allowing two million EU nationals to take part in it, warning such a move would create the worst crisis in British democracy for more than a century.

The Prime Minister’s plea came as the two leaders prepare for tonight’s final televised hourlong clash on BBC prime time, which could prove to be the Labour leader’s “last shot” to swing the dynamic of the election in his favour; all the polls thus far have pointed to a Tory victory.

Last night, Mr Johnson was accused by Labour of “running scared” after he turned down a one-on-one interview with ITV’s Julie Etchingham; he has also refused to submit himself to questioning by the BBC’s Andrew Neil, who last night goaded the PM and issued a personal challenge.

The Scot said: "There is no law, no Supreme Court ruling, that can force Mr Johnson to participate in a BBC leaders' interview. But the Prime Minister of our nation will, at times, have to stand up to President Trump, President Putin, President Xi of China. So, it was surely not expecting too much he spend half an hour standing up to me."

With time running out and the two main protagonists focusing on their core messages on Brexit and the NHS, the Conservative leader today sought to raise fears that a second EU poll could be rigged in favour of Remain.

In a letter to Mr Corbyn, the PM pointed out how academic research showed that 90 per cent of the two million EU nationals who would, under Labour’s plans, be eligible to vote in a second Brexit poll, would very likely back Remain.

Applying this to the likely turnout in a second referendum would mean, Tory HQ calculated, the Leave campaign could need nearly 500,000 more votes than it did in 2016 to win again.

Mr Johnson told the Labour leader that his plan to enfranchise two million EU nationals was a “sly attempt to undermine the result of the 2016 referendum” and would be profoundly undemocratic.

“No true democrat, even the most ardent supporter of Remain, could support your attempt to undermine the result of a democratically expressed vote,” he declared.

Cancelling the 2016 would be damaging enough, argued the Tory leader, but Labour’s policy would generate “incredible bitterness that might take decades to repair”.

He went on: “Imagine how people will feel if the biggest democratic exercise in our history is overturned because you gave two million EU citizens the power to reverse Brexit. It would alienate millions who already feel disenfranchised and ignored by our political system.

“We already face a real crisis of trust in our politics. It’s hard to imagine a policy that would do more to worsen this crisis than your plan.”

The PM, urging Mr Corbyn to reconsider his policy, added: “I fear if people vote to give you and Nicola Sturgeon control on Friday 13th and you do what you have said in your manifesto, it will create the worst crisis in democratic politics in over a century.”

Last night, the Labour leader appeared to recalibrate the timescale in which a future Labour Government would concede to Nicola Sturgeon’s demand and facilitate a second Scottish independence referendum.

Having previously said this would not happen in the early years of a Labour administration, Mr Corbyn told in his interview with ITV’s Julie Etchingham that he would not support another poll, noting: “It certainly wouldn’t be next year.”

He added: “I do not think independence for Scotland would be good for the people of Scotland in the sense of the economic future of Scotland.”

Labour’s Ian Lavery attacked the PM for refusing to do his own one-on-one interview with Ms Etchingham; his office said Mr Johnson would not be able to find time to do it.

“Boris Johnson thinks he’s born to rule and doesn’t have to face scrutiny,” declared the party’s Chairman.

“He’s running scared because every time he is confronted with the impact of nine years of austerity, the cost of living crisis and his plans to sell out our NHS, the more he is exposed,” he added.

ITV said it planned to go ahead and broadcast a programme on the Tory leader but this would “instead feature a profile of the Prime Minister using fresh interviews with other contributors and archive footage".

With less than a week to go both the Conservatives and Labour are far from certain of achieving the result they want next Friday morning; the issue of people voting tactically to keep a particular candidate from winning is a huge unknowable factor.

Last week’s Mullti-level Regression and Post-stratification[MRP] poll from YouGov, which used a massive sample of more than 100,000 people, suggested Mr Johnson was on course for a comfortable 68-seat Commons majority.

But Dominic Cummings, his key Downing St aide, warned the Tories against complacency, writing in his blog: “Trust me, as someone who has worked on lots of campaigns, things are much tighter than they seem and there is a very real possibility of a hung parliament.”

According to eminent psephologist, Professor Sir John Curtice from Strathclyde University, the key number is a seven.

A seven-point lead would point to the Tories winning a majority in the House of Commons. Given Mr Johnson could not, as Theresa May did, rely on the support of Northern Ireland’s Democratic Unionists to get his Withdrawal Bill through Parliament, then securing a majority, however small, is vital to getting Brexit done, the first part at least, on January 31.

Over recent weeks, the polls have suggested a Conservative lead of anything from 19 points to just six.

If Tory nerves are jangling, then Labour ones are probably jangling even more. Sir John has suggested there is “zero chance” of Labour getting a majority, which throws up the prospect of Mr Corbyn, if Labour becomes the largest party, having to rely on the support of Nicola Sturgeon’s likely-to-be-expanded contingent of MPs.

The MRP poll, news reports and evidence from the doorsteps suggest that Labour’s “red wall” across the Midlands and northern England is crumbling with traditional Labour supporters “lending” their votes to Mr Johnson to get Brexit over the line.

Pro-Brexit members of the Shadow Cabinet have been dispatched to these areas to bolster the party’s plan to secure a new exit deal from Brussels ahead of a second referendum.

But Sir John has warned that the Opposition has already lost Labour Leavers to the Tories in former strongholds.

Referring to tonight’s final head-to-head clash, one senior Conservative source noted: “Basically, Corbyn’s got one last shot. He has to change the narrative on Friday or he’s done.”

In other developments –

*Labour said new research showed the Tories had cut health and education spending by more than £37 billion since 2010. Andrew Gwynne, its campaign chief, said a “decade of brutal austerity” had “cut to the bone” health and education services.

*Mr Corbyn rejected claims he had made Labour a "refuge" for anti-Semites amid renewed accusations the party has failed to deal with the issue. Lawyers for the Jewish Labour Movement said 70 Labour staffers past and present had given sworn testimony to an official inquiry by the Equalities and Human Rights Commission into anti-Semitism in the party.

*The PM apologised “for any offence caused” for describing veiled Muslim women as looking like “letterboxes” and “bank robbers”.

*Greenpeace gave its rankings on the seven parties’ manifestos on plastics, placing the Green Party at the top, Plaid Cymru in second, the Liberal Democrats third, Labour fourth, the SNP fifth and the Tories sixth. Bottom was the Brexit Party.

*Mr Johnson boasted he could eat "the funny stuff" in one of I'm A Celebrity, Get Me Out Of Here...'s stomach-churning challenges.