He responded to a letter written to him by deputy Labour leader Lucy Powell in which she said she feared he was being “played” by Reform and would allow them to win by splitting the Labour vote.
Polanski said her claims were “absolutely absurd”.
He said: “The Green Party are the only party who can stop Reform and to have the Labour Party trying to wade in, trying to make themselves relevant, demonstrates what a difficult week they’ve had and what a difficult year they’ve had since gaining power.”
At the 2024 general election, Labour won the seat with more than half of the vote – 18,555. Reform UK came second on 5,142 votes, narrowly beating the Greens with 4,810.
The by-election, which will take place on 26 February, was triggered by the resignation on health grounds of former Labour MP Andrew Gwynne.