Unlike the leadership and deputy leadership contests, Labour insiders are sharply divided over who will win the party's London mayoral nomination. Tessa Jowell, long the frontrunner, and Sadiq Khan, the former shadow justice secretary, are both spoken of as potential victors when the result is announced this afternoon at the Royal Festival Hall. "Sadiq's got it. The Corbynites will deliver it for him," one shadow cabinet minister told me. "Tessa will still win - and that's some consolation," another said. David Lammy, a rival mayoral candidate, was said by sources to have predicted that Khan would win.

Jowell, the former culture secretary, opened up a commanding lead after declaring her bid in November 2014. But sources say Khan has closed the gap in recent weeks as he picks up support from left-leaning voters. As well as endorsements from Unite and the GMB, his anti-war record, opposition to the welfare bill and nomination of Jeremy Corbyn have aided him. But others say that, despite the ideological gulf between them, Jowell is winning support from Corbynites. "I’ve spoken to people who are voting enthusiastically both for Jeremy and for Tessa," Wes Streeting, the newly elected MP for Ilford North, told me. "They have both run very optimistic, hopeful, positive campaigns." Sources suggest that some Corbyn supporters are unaware of Jowell's Blairite roots and are attracted by her Olympics record and personability.

Both campaign teams are claiming victory. "It's very close but I think it's advantage Sadiq," one Khan campaign source said. "Tessa should have walked away with this race. That she hasn't is a testament to how strong the campaign has been." A Jowell aide said: "We're very confident, we're very pleased with the campaign that we've run. All of our evidence points to a good result." Her team are confident that her lead among Labour members will offset any advantage Khan has among affiliated supporters. "We're performing well in all three sections," a source said.

Diane Abbott, the candidate most closely aligned with Corbyn, is expected to finish third. Some in the party are troubled by her failure to break through. "People who are voting Jeremy tell me that they think Diane just isn't up to it," one MP said. "There's a lot of unconscious racism and sexism." Whoever wins the nomination has a good chance of securing the mayoralty. London is a Labour city (the party won 45 of its 73 seats and 44 per cent of the vote at the general election) and the likely Conservative candidate, Zac Goldsmith, cannot match Boris Johnson's popularity.

But some Jowell supporters fear that a Corbyn leadership would inflict collateral damage on her. "She’ll get killed, she’ll get killed," John McTernan, Tony Blair's former director of political operations, told me. "The Labour brand is a single brand and it’s a brand set by the parliamentary leader. There’s no Tessa Labour brand."