Chris Williamson is suing Labour after he was re-suspended as one of the party’s MPs for claiming it was “too apologetic” on antisemitism.
The Derby North MP is attempting to get readmitted to the party by launching legal action, which he hopes will “overturn the unconstitutional decision” to reimpose his suspension.
In a tweet, he wrote: “I’ve dedicated my life to the Labour Party since I was 19 years old, and I’m 63 next month.
“Remember, comrades: UNITY IS STRENGTH!”
A source close to Mr Williamson said that legal papers have already been filed in court.
But Labour appears confident it followed rules and there will not be successful grounds for a challenge.
Mr Williamson, who is a strong supporter of Jeremy Corbyn, was suspended in February after footage emerged of him telling a Momentum meeting that Labour was being “demonised” over the antisemitism crisis.
He also said he had celebrated the resignation of MP Joan Ryan, who quit Labour over the handling of antisemitism and bullying complaints.
In July, his suspension was lifted by Labour’s National Executive Committee after he received a formal warning.
The party was then forced to make a U-turn when more than 100 MPs and peers launched an open revolt calling for Mr Williamson to be expelled.
At the time, the MP said he would do his “utmost to fight” the re-suspension.
The case is now being considered by a new antisemitism panel.
A Labour Party spokeswoman said: “Chris Williamson is suspended from the Labour Party pending a hearing of the National Constitutional Committee, Labour’s highest disciplinary body.
“We do not give a running commentary on individual cases.”
A campaign group has attempted to crowdfund £75,000 to support Mr Williamson’s legal action.
At the time of writing, it has managed to raise over £12,000.