Keir Starmer has hinted at a 'rainbow alliance' to fight the Tories at the next general election - which he expects to happen within two years.
The Labour leader refused to rule out making a pact with other parties such as the Lib Dems and the Greens in order to maximise the chances of defeating Boris Johnson.
Labour was seen as having helped deliver victory for the Lib Dems in the recent Chesham & Amersham by-election by carrying out minimal campaigning.
But such a strategy could also revive a favourite Tory attack line about the possibility of Labour teaming up with the SNP to form a government - a prospect weaponised effectively by David Cameron in 2015.
Keir Starmer (right) has hinted at a 'rainbow alliance' to fight Boris Johnson (left) at the next general election - which he expects to happen within two years
In an interview with ITV's Robert Peston, Sir Keir was asked if an electoral alliance could be on the cards given the scale of the 'deficit' he has to make up against the Tories.
'We've got to do a lot more work, you know, obviously going into the next general election. There will be a question of what we do,' he said.
'There's a majority broadly against the Tories in the country and obviously we'll have to see how we go into the next general election, but, you know, the rules are what they are, but we're rebuilding our party, putting out our arguments and going hell for leather for that election - probably May 23, I suspect.'
Sir Keir said he believes the 'road will run out' for Mr Johnson as the public believe in 'integrity, honesty and accountability'.
He claimed Mr Johnson had been 'lucky' due to the pandemic, with the Government able to address fears over the virus and job security through the vaccine programme and the furlough scheme.
But with most people now jabbed and the furlough scheme winding down, Sir Keir suggested the public's attention will turn to other aspects of the PM's record.
There was 'government by soundbite' and 'mounting promises that are just broken' on issues including the Northern Ireland Protocol, the size of the army, overseas aid, tax and social care.
Labour was seen as having helped deliver victory for the Lib Dems in the recent Chesham & Amersham by-election (pictured) by carrying out minimal campaigning
'I don't buy into this argument that character just doesn't matter. I do think that in the end integrity, honesty and accountability do matter and I think there will come a point at which the road will run out for Johnson on this.'
The Labour leader, an Arsenal fan, praised England football manager Gareth Southgate for showing 'far more leadership than the PM' and joked that he would have pundit Gary Neville in his shadow cabinet as 'he's been very critical, rightly, of the Prime Minister'.
Sir Keir also took a swipe at Mr Johnson's appearances supporting England, saying: 'I've been an England fan all my life and I don't wear my shirt over my shirt and tie because real fans never do that – it's just a no-go area.'