Alex Rowley has vowed to stay on as Scottish Labour’s acting boss despite being exposed as backing the left-wing candidate for the leadership.
As the caretaker, Mr Rowley is supposed to be neutral in the race between Anas Sarwar and Richard Leonard for the party’s top job.
But he was recorded declaring his support for the pro-Corbyn Mr Leonard in a private conservation at the Brighton conference.
Mr Rowley, a Fife MSP and former leader of the kingdom’s council, was also accused of letting slip the existence of a plot to replace Kezia Dugdale before 2021.
It has triggered an outbreak of in-fighting, with supporters of Mr Sarwar condemning the left’s manoeuvres.
Senior Labour MSP Jackie Baillie said the conspiracy to oust the then leader, who quit suddenly last month, will “absolutely appal” members is a “complete betrayal” of everything the party stands.
“Kez was elected with a huge majority and it will infuriate members to learn that some MSPs were working behind the scenes to undermine her and replace her with Richard Leonard,” she added.
“Richard must urgently answer questions about his role in this plot.”
A spokesman for Mr Leonard denied he had anything to do with plotting against Ms Dugdale.
“If there were any attempts to undermine Kezia, they did not involve Richard and he knew nothing of them,” he said.
“Any statements that he did are completely false.”
In extracts published in The Sun, Mr Rowley told a student and political activist: “Although I’m neutral in the leadership I also believe that Richard Leonard has everything that we need to win in 2021. I really do.
“So when, to our surprise, the job became vacant, it just seemed to me that from a Left perspective, the person that was most suited to do that job would be Richard Leonard.”
He has publicly said he “won’t be supporting anyone”.
In the conversation, he also predicted Mr Sarwar could not last more than four years in the role if he won.
A Scottish Labour spokesman said: “Alex was having what he believed to be a private conversation with a student and a political activist.
“He has not and will not publicly back a candidate.
“He has no intention of relinquishing his role as interim leader of the Labour Party in Scotland until after the current leadership contest is over.”