Johnson called the snap election - the third in Britain in nearly five years - last month to try to get a parliamentary majority which would enable him to secure backing for his divorce deal.
Voting takes place next Thursday. The Britain Elects poll aggregator puts the Conservatives on 42 per cent, Labour on 33 per cent and the Liberal Democrats on 13 per cent.
The Greens and the arch-eurosceptic Brexit Party were both on three per cent.
Ahead of Friday evening’s debate former prime ministers
Tony Blair and John Major launched unprecedented interventions, calling for people to vote tactically to help ensure 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, addressed a rally for another poll in London
Both want Britain to remain in the EU.
Major gave his backing to several candidates thrown out of the Conservative ranks for rebelling over Brexit. “Let me make one thing crystal clear, none of them left the Conservative Party, the Conservative Party left them,” he said via video-link.
“Were I resident in their constituency I would vote for them.”
Asked about the comments, Johnson insisted his party retained ‘a very broad spectrum of views’ and noted that all Tory candidates had taken a vow to back his deal.
But in a blow to Johnson, a senior British diplomat in the US quit on Friday, criticising the government over Brexit.
Alexandra Hall Hall said she could no longer ‘peddle halftruths’ on behalf of political leaders she did not ‘trust’, according to CNN, which obtained a copy of her resignation letter.
Earlier on Friday, Johnson came under fire for avoiding a set-piece television interview that all other major party leaders have already subjected themselves to.
The Prime Minister has so far declined to undergo an uncomfortable grilling from Andrew Neil, who is one of the BBC's top political interviewers, with less than a week to go until the election.
Britain’s Prime Minister Boris Johnson (centre) and main opposition Labour Party leader Jeremy Corbyn (left) participate in the BBC Prime Ministerial leaders debate presented by BBC’s Nick Robinson (right), at the studio in Maidstone, Kent, on Friday