Ian Lavery, one of Mr Corbyn’s closest allies, claimed that members were so ‘demoralised’ that they are considering who would stand against Sir Keir.
Former party leader Mr Corbyn was suspended last month for claiming that anti-Semitism in Labour was ‘dramatically overstated for political reasons’.
His 19-day suspension was lifted on Tuesday by a panel of the party’s ruling national executive committee (NEC).
Ian Lavery, one of Mr Corbyn’s closest allies, has said that Sir Keir Starmer faces a leadership challenge if he continues to prevent Jeremy Corbyn from sitting as a Labour MP
But Sir Keir refused to restore the whip, meaning he must continue sitting as an independent MP.
There has been an outcry at the decision from Mr Corbyn’s supporters, and Mr Lavery said some were even discussing a leadership challenge.
‘People are scrambling now for the copies of the rule book to see what would be required if indeed it came to that stage,’ the Labour MP told Times Radio.
‘I don’t think there’s an appetite for a legal challenge but I think that it’s at that stage which I’m sure you’ve seen a time or two yourself when people and members are that demoralised that they might wish to at least understand how we can approach a potential leadership challenge.
‘People are not saying this is a regular topic in the tea room, it probably isn’t.
‘What I will say is that people now are wondering how, if there was a decision made, how and who could stand against Keir Starmer.
‘I think it’s far too early. I would prefer Keir Starmer to just focus on attacking the Tories, get Jeremy back in the party and let us move forward as a unified party.’
Mr Lavery is seen by allies as a prime candidate to lead a challenge, but he denied that it was something he would pursue at the moment.
He also accused Sir Keir of pursuing ‘a personal and political vendetta’ against his predecessor and turning the party into a ‘tinpot dictatorship’, in comments to the news website HuffPost.
Sir Keir refused to restore Mr Coybyn’s whip, meaning he must continue sitting as an independent MP
Meanwhile, a Labour peer resigned from the party yesterday after nearly 50 years over its treatment of anti-Semitism and Mr Corbyn’s readmission.
Lord Desai, an economist who was made a peer in 1991, wrote to The Times: ‘I have been very uncomfortable and slightly ashamed that the party has been injected with this sort of racism.
‘Jewish MPs were abused openly, and female members were trolled. It is out-and-out racism.’
Responding to Lord Desai’s resignation, Gideon Falter, chief executive of Campaign Against Antisemitism, said: ‘It is a courageous decision reminiscent of those MPs and peers who quit the Party under Jeremy Corbyn in disgust.’
Mr Corbyn instructed solicitors on Thursday to begin pursuing a case against his party after Sir Keir refused to return the whip.
He has been told by Nick Brown, Labour’s chief whip, that he will be suspended for three months – and could return as a Labour MP after that depending on his behaviour.
Mr Corbyn was suspended over comments he made in response to an Equality and Human Rights Commission (EHRC) report which found that Labour had unlawfully discriminated against Jewish members.