Boris Johnson came under fire on Friday for avoiding a set-piece television interview and faced accusations of misleading Britain about his Brexit deal, as he prepared to face a final general election debate with his main challenger.

Four other major party leaders have already subjected themselves to an uncomfortable grilling from Andrew Neil, who is one of the BBC's top political interviewers.

But the prime minister has so far declined to do so, with less than a week to go until Britons go to the polls to elect a new parliament for the third time in four years.

The Conservative party leader brushed off fresh questions about the issue as he toured southeast England before the second and last TV showdown with Jeremy Corbyn at 20:30 GMT.

"I'm the only prime minister to have done not one but, by tonight, two head-to-head debates," the Conservative party leader Johnson told reporters at a poster launch in Kent.

"I've done 118 sit-down interviews with journalists... We can't do everyone," he added.

Former Sunday Times editor Andrew Neil said Johnson faced questions of trust, and it was his job to "scrutinise and hold to account those who would govern us".

"We've always proceeded in good faith that the leaders would participate. And in every election they have. All of them. Until this one," he said.

'Cold, hard evidence'

Labour Party leader Corbyn meanwhile claimed he had obtained leaked documents proving Johnson was "deliberately misleading the people" about his Brexit divorce deal.

He said finance ministry papers suggested there would be customs declarations and security checks between mainland Britain and Northern Ireland, contrary to what he had claimed.

"This is the cold, hard evidence that categorically shows the impact Johnson's damaging Brexit deal will have on large parts of our country," the veteran socialist said.

"This drives a coach and horses through Boris Johnson's claim that there will be no border in the Irish Sea," he added, suggesting the Tories could be withholding more details.

Corbyn has previously accused Johnson of cooking up a secret deal with US President Donald Trump to sell off the state-run National Health Service, citing unredacted documents.

Both Trump and Johnson deny the claim.

The Conservatives claimed the Treasury paper was an "immediate assessment, not a detailed analysis of the deal" and was not used for decision-making purposes.

Party co-chairperson James Cleverly said Corbyn's leaked documents "don't back up his wild conspiracy theories".

Labour is proposing to renegotiate a softer form of Brexit, then put it against staying in the European Union in a second referendum in which all UK-based EU nationals would be able to vote.

Cleverly said that by proposing to extend the franchise, Corbyn was trying to "rig" the result.

Johnson called the snap election to try to get a parliamentary majority which would enable him to secure backing for his deal for Britain to leave the EU.

Voting takes place next Thursday. The Britain Elects poll aggregator puts the Conservatives on 42 percent, Labour on 33 percent and the Liberal Democrats on 13 percent.

The Greens and the arch-eurosceptic Brexit Party were both on three percent.

Major, Blair intervention

Former prime ministers Tony Blair and John Major were meanwhile set to intervene in the campaign to call for a second referendum on Brexit.

Major, a Conservative who was in power from 1990 to 1997, and Labour's Blair, who ousted him and was in Downing Street until 2007 were expected at a rally for another referendum.

Both want Britain to remain in the EU.

Major was to give his backing to candidates thrown out of the Conservative ranks for rebelling over Brexit, according to excerpts of his speech released in advance.

"None of them has left the Conservative Party; the Conservative Party has left them," he was to say, describing Brexit as the "worst foreign policy decision in my lifetime".

"It will make our country poorer and weaker. It will hurt most those who have least. Never have the stakes been higher."

Mocking Johnson's campaign slogan, Blair was to say: "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."