Jeremy Corbyn is being attacked by rivals after winning a key endorsement from a trade union.
In a surprise move, Unison said it had decided to nominate Corbyn and put Yvette Cooper as its second choice. Many in the party believed the endorsement would go to either Cooper or Andy Burnham.
Labour insiders from rival camps told BuzzFeed News that Corbyn could not keep up the momentum. With six weeks to go before the leader is announced, sources insisted the 66-year-old would not be able to sustain the soaring levels of support he is enjoying.
Corbyn is now the bookies' favourite to become Labour leader – despite odds as long as 100/1 at the start of the race.
But one insider said: "We have reached peak Jeremy." Another told us: "Jeremy is clearly getting through to people but we have a long way to go yet. He hates answering personal questions and that will be hard to keep up."
There was even speculation in Westminster that the reason Corbyn pulled out of a planned interview on ITV1's This Morning on Wednesday was because they could not guarantee there would be no questions on his personal life.
But an ITV spokesperson said the programme was always "subject to change" and they hoped to have Corbyn on the show next week.
The MP for Islington North, who has never sat on the front bench, is winning many voters over by rejecting austerity and answering questions with straight answers.
Dave Prentis, Unison's general secretary, said Corbyn's message had "resonated with public sector workers who have suffered years of pay freezes [and] redundancies".
Corbyn welcomed Unison's nomination, saying: "We are building a movement for a modern, kinder Britain, and I look forward to working with Unison members to achieve that."
But the Cooper and Burnham camps played down the Unison endorsement, both saying they never expected to get it.
A source close to Cooper said Burnham's team must be "gutted" because he had spent years courting the union. "We're getting lots of support from Unison members though and working with them on the campaign," they added.
But Burnham supporters insisted Cooper's camp had put a lot of work into winning Unison's nomination and must be hugely disappointed.