Labour Mayor of London Sadiq Khan is to share a platform with Prime Minister David Cameron at a Remain rally later.
They will jointly launch a Britain Stronger In Europe battle bus, despite Labour leader Jeremy Corbyn refusing to campaign with Mr Cameron.
It comes after Mr Cameron had criticised Mr Khan’s “poor judgement” during the London mayoral election.
Mr Khan defended his decision to campaign publicly with the Tory PM and said it was in London’s interest.
Is it in London’s interest for me to hold grudges? Is it in London’s interests for the mayor of London to be at permanent war with the Conservative prime minister?” Mr Khan told ITV’s Peston On Sunday.
“We’re never going to be best friends, but what is important is that the mayor of London argues the case for London and for Londoners to remain in the European Union.
“This debate is far more important than David Cameron or me.
“It’s about our city’s future and country’s future.”
During London’s mayoral election race, Mr Cameron repeatedly attacked Mr Khan for having a history of sharing platforms with extremists, which Mr Khan rejected.
At Prime Minister’s Question Time in the Commons some Labour MPs denounced Mr Cameron as a “racist” when he attacked Mr Khan’s judgment.
In other EU referendum developments:
The World Trade Organisation’s former director-general has warned that the UK economy risks a “huge blow” if it relies on the agency’s global trading rules in the case of an EU leave vote.
Labour’s police and crime commissioners have signed an open letter urging a Remain victory, because they say leaving the EU would risk national security.
On Sunday, senior Tory ministers Michael Gove and Boris Johnson challenged Mr Cameron on immigration, saying he must accept the failure of the government’s manifesto pledge to reduce migration into the UK. They said the pledge was “corrosive of public trust” while Britain remained in the EU.
And former Labour prime minister Tony Blair said leaving the EU would create “an enormous economic problem”.
Sunday’s row over the migration target came as Conservative in-fighting over the EU referendum intensified.
Mr Cameron and most of his cabinet are campaigning for a vote to stay in the EU, but about half of his MPs support an exit.
Conservative Leave campaigner Andrew Bridgen had told BBC Radio 5 live’s Pienaar’s Politics that as many as 50 Tory MPs were ready to back a vote of no-confidence in the prime minister over his handling of the debate.
But leading “Out” advocate and Labour former minister Frank Field has condemned the “Brexit” camp’s personalised onslaught on Mr Cameron.
Mr Field told BBC Radio Four’s Westminster Hour: “The first mistake I think our side has made has been to question the prime minister’s integrity.
“I think it’s a mistake to put your sticky fingers into people’s souls and say they’re debasing public life.
“I think it would be very good for our Out campaign to actually concentrate on issues and not on the integrity of the other side.”
Number 10 dismissed the personal attacks as a “distraction” intended to move attention away from the economic arguments for remaining in the EU.