Ken Livingstone claims Labour's National Executive Committee (NEC) banning members from voting in their leadership election is a "scam".
Members who joined after January 12 will not be allowed to vote for the leader of the party, which is currently between Jeremy Corbyn, Angela Eagle and Owen Smith. The membership price has also increased from £3 to £25.
"It's completely undemocratic and it's all a bit of a scam because the right-wingers obviously waited for Jeremy and a couple of his supporters to go downstairs and talk to the media, and it all got pushed through."
"I suspect someone might challenge this in the court," he told Paul Ross. "I sincerely hope the members of the NEC will recognise this is a ridiculous bureaucratic mistake, and profoundly undemocratic."
Many MPs had previously called for Jeremy Corbyn's resignation after the EU referendum, but Livingstone believes MPs need to accept that members are happy with his leadership.
"He wasn't going to go round the country lying like Cameron and Johnson did [in the EU campaign]," he added. "He was telling people the truth."
"Labour MPs need to recognise that the party want Jeremy Corbyn," he explained. "They should just get behind him they're out of touch in their little Westminster bubble, they don't realise that people believe he's a regular guy and is someone they trust."