Angela Eagle will delay mounting a bid to unseat Jeremy Corbyn as Labour leader
A source close to the former shadow business secretary told The Huffington Post UK: "There's still time for Jeremy to back down. And see sense."
Eagle had been expected to launch her leadership bid at 3pm on Thursday afternoon.
As Boris Johnson sensationally pulled out of the Conservative leadership battle, the source added: "Today's a day for the Tories. But she will stand."
A source close to Corbyn told HuffPost of her decision: "Where Eagles Daren't...that's the movie title. Even she's not sure if she should run. Shes's a second rung Brownite."
This morning, Corbyn admitted he was having a "a torrid few days" but gave no sign he was prepared to resign.
And he reminded his critics of the "mandate" given to him by Labour members when he was elected in 2015. "I was honoured to be elected leader by over a quarter of a million people. That is, by the way, substantially more than the entire electorate that will have the right to pick the Conservative Prime Minister this Autumn," he said.
Last night at a rally organised to show support for Corbyn, shadow chancellor John McDonnell accused Labour MPs of trying to "lynch" their leader.