Jeremy Corbyn's solicitors have written to Labour calling for his suspension as one of the party's MPs to be lifted, the BBC has been told.
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.
He said his predecessor's remarks had "undermined trust" in the party within the Jewish community.
Mr Corbyn has been told his suspension from the parliamentary party will last for three months, though this can be reviewed.
The BBC understands Mr Corbyn's lawyers criticised the lack of a specific timescale when the suspension took place.
And he could face a further investigation under the parliamentary party's rules.
BBC political correspondent Iain Watson has seen a letter from 15 members of Labour's ruling National Executive Committee (NEC) who support Mr Corbyn to the most senior party official, alleging political interference in the former leader's case.
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 confirmed that a solicitor's letter had been received but no legal proceedings had begun.
The uneasy truce between the current leadership and the supporters of the former leader ended when Sir Keir Starmer refused to readmit Jeremy Corbyn to the Parliamentary Labour Party.
And so, a battle has begun.
Another former Labour leader and ex-prime minister, Gordon Brown, has called for Mr Corbyn to apologise for the comments leading to his suspension, in which he said 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 NEC 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.
Another former Labour leader, ex-Prime Minister Gordon Brown, has called for Mr Corbyn to apologise for his comments.
And former Labour deputy leader Tom Watson, who served under Mr Corbyn, told the BBC's Newscast podcast: "It would just be a lot easier if there could be more contrition [on Mr Corbyn's part], rather than lawyers' letters flying around."