Andy Burnham's allies have accused Labour leadership rival Wes Streeting of deliberately trying to undermine the Greater Manchester Mayor's Makerfield by-election campaign.
Mr Streeting, who threw his weight behind Mr Burnham contesting the Brexit-backing seat, appeared to put his soon-to-be opponent in a difficult position after setting out a path for the UK to rejoin the European Union.
Speaking at an event in London on Saturday, the ex-Health Secretary said: “Britain’s future lies with Europe, and one day back in the European Union.”
Mr Burnham, who expressed a similar view last year's Labour Party conference, is offering a somewhat different message to voters in Makerfield.
However, Labour insiders fear a row over Brexit is now a gift to Reform UK in Makerfield.
A senior source told The Times: “I despair. It’s like a gift to Farage. What Wes did reeks of desperation and selfishness.”
An ally of Mr Burnham added: “Wes’s only hope at becoming the next leader is for Andy to lose the by-election. [Mr Streeting’s] comments … are counterproductive to Labour winning this by-election."
Despite concerns from Mr Burnham's inner circle, allies of the ex-Health Secretary pointed out the Greater Manchester Mayor expressed a similar view on Europe less than nine months ago.
Wes Streeting has committed to reversing Brexit as part of his leadership campaign
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“I thought they agreed on this,” the source said.
“The problem is that we’re so worried that there might be people who disagree with us if we do something that we end up doing nothing."
Mr Burnham tonight desperately tried to U-turn on his push to reverse Brexit.
A spokesman for the Greater Manchester Mayor told The Telegraph: “Andy is fully focused on working hard for every vote in Makerfield so he can represent them in Parliament.
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Andy Burnham has to win a by-election in the Greater Manchester constituency, Makerfield, before he can make a leadership bid for the top job
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“Andy is not standing on a national manifesto at this election; he is standing to make a difference for the people of Makerfield and to bring the change he has delivered in Greater Manchester to the national stage.”
However, Reform UK is already sensing an opportunity to put pressure on Mr Burnham over Brexit, with Mr Farage hoping that the issue will cut through with the Leave-voting constituents of Makerfield.
He said: “‘Open borders Burnham’ wants to give 500 million people the right to move here without any thought on how this would affect schools, doctors and dentists.
“It’s quite clear that this by-election is now a choice between Reform, who want to stay outside of the EU, control immigration and deport all illegals, and Labour, who want to rejoin the EU and open the borders.”
Culture Secretary Lisa Nandy, who represents the neighbouring Brexit-backing seat of Wigan, also urged her political friends against reopening the Brexit debate.
Speaking to GB News, Ms Nandy said: “To respond to that with reentering the European Union seems to me to be a fundamental misreading of what people are asking us for.
“If going back into the EU was the answer to the problems that people have in their lives, then we would be telling people in towns like mine that everything was fine in 2015. It absolutely wasn't."
Labour peer Maurice Glasman also told The People's Channel: "We've resolved the Brexit issue.
“For him to say, rejoin an ailing institution, an economically stagnant institution, was ridiculous.
“The other thing that they don't see is that the AfD and Le Pen are going to win the next elections in France and Germany.
“I mean, this is bonkers."
Fifty-five per cent of Britons are in favour of Britain rejoining the EU, according to a YouGov poll in April.