Andy Burnham was dropped by a prominent business backer on Wednesday over his views on single sex spaces.
Former Dragons' Den star Duncan Bannatyne said he could no longer support Mr Burnham's bid for power following revelations in the Daily Mail that he supports allowing men who self-identify as female to use ladies' toilets.
The leisure tycoon, who has donated large sums to Labour in the past, branded the Greater Manchester mayor a 'cretin' for failing to support the preservation of single sex spaces.
Writing on X, formerly Twitter, the entrepreneur, estimated to be worth £500million, said: 'Last week I decided to throw my support behind Andy Burnham for the leadership of the Labour Party and on to Prime Minister.
'But I cannot do that now that I find out he says transwomen, ie men, should be allowed to use the ladies [sic] toilets. I can never support anyone that does this. Biological women must have single sex spaces.'
In a leaked recording from 2022 obtained by the Daily Mail, Mr Burnham claimed the idea that single sex spaces such as toilets should be protected for biological women was a 'minority view'.
He dismissed the idea that women's safety could be put at risk by the presence of biological males. And he hit out at 'supposed feminists' trying to start 'culture wars' by demanding protection for single sex spaces.
His office has so far failed to clarify whether his views have changed.
Former Dragons' Den star Duncan Bannatyne (left) said he could no longer support Mr Burnham's bid for power following revelations in the Daily Mail that he supports allowing men who self-identify as female to use ladies' toilets
The Manchester mayor has signalled he will mount a leadership bid if he wins the Makerfield by-election
Andy Burnham displays his support for the LGBTQ community at last year's Manchester Pride Parade
However, last year he was critical of interim guidance from the Equality and Human Rights Commission following a landmark supreme court ruling that single sex spaces should be reserved for people of that biological sex.
On Wednesday he was urged to come clean about whether his views have changed – or if he still believes biological males who identify as transgender should be allowed to use single-sex spaces reserved for women, such as toilets and changing rooms.
Campaign group Sex Matters said: 'Women's rights are not a culture war. Polling shows most people want single-sex provision, and don't think men who identify as women should be allowed into women-only spaces – this is not a 'minority view'.
'Andy Burnham must now state clearly whether or not he supports the Equality Act and accepts that when it comes to women-only spaces and services, men who identify as women are not women, and don't have the right to enter.'
The group's co-founder Maya Forstater said: 'One who aspires to be PM doesn't have the luxury of staying silent on a matter of great importance to many women and to the rule of law.'
Mr Bannatyne, left, was one of the Dragons on the investing show from 2005 to 2015.