Labour chiefs fear former minister Kate Hoey will defect to the Brexit Party to become Nigel Farage’s first MP.
Ms Hoey, 73, announced she would be standing down at the next General Election after representing Vauxhall in Central London for 30 years.
She fell out with her local party last year after voting with the Tories on Brexit. Activists have since been trying to oust her.
Labour MPs became suspicious when Ms Hoey was spotted meeting Brexit Party chairman Richard Tice in the House of Commons10 days ago.
One said: “That sparked our worries she will become the first Brexit Party MP.
“Kate was facing deselection anyway and that may have finally tipped her over into defecting.”
Earlier this month, Ms Hoey announced that she was retiring from politics in a passionate letter.
She said: "I will carry on until a general election serving with the energy, honesty and integrity that I have tried to bring to public service my whole life."
Ms Hoey, who famously campaigned alongside Nigel Farage in favour of leaving the EU, had previously said she would not fight the 2020 election but when the 2017 snap poll was called she was persuaded to stay on.
It comes almost a year after she lost a no confidence motion tabled by her Vauxhall Constituency Labour Party.
Those who attended the meeting last July voted unanimously for a call to oust her last night in a damning motion that accused her of "ignoring" constituents' views, "collaborating" with Hard Brexit Tories and "colluding with Nigel Farage".
Ms Hoey is a long-term eurosceptic who voted against all the Maastricht and Lisbon treaties.
But it is not just her stance on Brexit that meant the former minister often clashed with her local members.
Despite representing one of the most inner city seats she is a former chair of the Countryside Alliance and is in favour of fox hunting.
She attacked Labour's decision to ban handguns following the Dunblane massacre.
She is also in favour of grammar schools which puts her at odds with lots of Labour Party members.
She has a patchy voting record on gay rights and abortion.
Despite spouting views which are at odds with the majority of her constituents, Ms Hoey held the seat in 2017 with 57 per cent of the vote.
