Labour MPs turned on Andy Burnham after he agreed to share a platform with the Greens just weeks after the local elections.
The Greater Manchester Mayor will appear at a conference for the ‘progressive majority’ with former Green Party leader Caroline Lucas.
But Labour MPs reacted with fury that he would appear alongside their rivals just after a bitter local election battle.
The Greens are expected to make incursions into traditional Labour territory and win a number of council seats from Sir Keir Starmer’s party in the local elections.
They are expected to take Labour strongholds such as Hackney, Lewisham, Lambeth and Waltham Forest in east London.
They could even make gains in Camden, in Sir Keir Starmer’s home borough, as well as Greenwich, Southwark and other Labour heartlands.
While Zack Polanski’s party has dipped slightly in the polls after a number of Green candidates made anti-Semitic comments, it still represents a major threat.
Labour MPs said they were unimpressed by Mr Burnham’s decision and said it would give people cover to vote for the Greens by suggesting they were aligned with Labour.
Labour MPs turned on Andy Burnham after he agreed to share a platform with the Greens just weeks after the local elections
One Labour MP said of the event: ‘I’m not impressed. There will be lots of ex-council leaders and councillors who will also be pissed off with that.
‘It gives people the green light to vote for the Greens by implying a more Left-wing Labour Party is closer to the Greens than KS’s [Keir Starmer’s] Labour.’
Another Labour MP said: ‘The Greens are getting the airtime Burnham craves.
‘He did nothing to tackle antisemitism when he was an MP, pretending to be Left when he was a Blairite minister.
‘He’s taking the same unprincipled approach snuggling up to the Greens now.’
Following Labour’s expected drubbing in the local elections, Sir Keir is likely to come under pressure from MPs who wish to replace him.
Mr Burnham is seen as one of the likeliest contenders despite the Prime Minister blocking him from standing in a by-election earlier this year.
But his decision to appear with the rival Green party will upset Labour MPs who are fighting to keep their councils.
The Left-wing group Compass and Progressive Economic Forum event will be held in London on Saturday, May 30 to ‘mobilise the progressive majority’.
‘Speakers from some of the biggest progressive parties and organisations will come together to figure out: how [do] we turn the country’s progressive majority into power,’ it says.
Speakers will include Left-wing MPs and figures from Labour, the Greens and the Liberal Democrats.
Clive Lewis, who is on the Left of Labour and has previously offered to stand down to make way for Mr Burnham, will also be speaking.
Miatta Fahnbulleh, the communities minister, is also due to speak at the event as well as several Lib Dems, including Vince Cable.
The event says it will bring together ‘progressives of all parties (and none)’ for a day of ‘debate, disagreement and deliberation’.
Compass’s founder, Neal Lawson, has long campaigned for a cross-party progressive alliance but is also a prominent supporter of Mr Burnham.
A source close to Mr Burnham said: ‘Andy isn’t sharing a platform and it’s ridiculous to suggest he is supporting other parties – he is advancing progressive ideas to help the Labour Party.
‘Do people not want anyone sharing ideas or having a debate about progressive policies?’
