The BBC has been told Jeremy Corbyn’s solicitors have written to the Labour Party calling for his suspension to be lifted.
The former leader was readmitted as a party member on Tuesday, after a short suspension for his reaction to a report into anti-Semitism in Labour.
But his successor, Sir Keir Starmer, has refused to let him sit as a Labour MP, saying his remarks had “undermined trust” with the Jewish community.
The decision will remain under review.
BBC political correspondent Iain Watson understands the letter from Mr Corbyn’s lawyers questions whether procedures had been properly applied when the decision was taken.
Our correspondent understands supporters of the former leader who sit on Labour’s ruling body, the National Executive Committee, will release a statement too, saying the disciplinary processes of the party had been undermined by Sir Keir’s actions.
But many MPs and Jewish groups stand by the decision taken by the new leader.
The parliamentary chair of the Jewish Labour Movement, MP Margaret Hodge, said the move by Sir Keir had stopped her from resigning from the party.
She told BBC Radio 4’s Today programme: “[Mr Corbyn] is not a victim. We have been the victim of the anti-Semitism.”
The Labour Party has been contacted for a response.
The uneasy truce between the current leadership and the supporters of the former leader ended yesterday when Sir Keir Starmer refused to readmit Jeremy Corbyn to the Parliamentary Labour Party.
And today, a battle has begun.
Another former Labour leader and ex-prime minister, Gordon Brown, has called for Mr Corbyn to apologise for the comments which led to his suspension – saying the the scale of anti Semitism had been dramatically overstated.
But Mr Corbyn shows no signs of doing so.
In the coming week, Sir Keir will want to focus on the government’s handling of the pandemic and on the spending review.
But, like Labour leaders before him, he is now facing a serious conflict in his own ranks.
Mr Corbyn was suspended at the end of October after a damning report by the Equality and Human Rights Commission (EHRC) into Labour’s handling of anti-Semitism allegations during his tenure as leader.
But it was the former leader’s response to the report – saying the scale of anti-Jewish abuse had been “dramatically overstated” by his political opponents – that led to the decision by Labour’s general secretary David Evans to suspend him after 54-years of membership.
A panel of the party’s ruling body, the National Executive Committee, decided on Tuesday to readmit Mr Corbyn as a member, but this did not mean he would automatically be reinstated as a Labour MP.
On Wednesday, Sir Keir decided not to allow his predecessor to represent Labour in the Commons – known as stripping him of the party whip – meaning while Mr Corbyn remains an MP, he will sit as an independent.