Labour's big city mayors can show the party the way back into power, Mayor of London Sadiq Khan is due to tell its annual conference in Liverpool.
In his first conference speech since entering City Hall in May, Mr Khan - tipped as a possible future Labour leader - will warn the party that it must focus its attentions on winning power and insist that settling for opposition "will never, ever be good enough".
Mr Khan will congratulate Mr Corbyn and say that "after the election this summer the leadership of our party is now decided".
But he will make clear that Labour must now devote its efforts to seizing national power rather than to internal party power struggles.
Mr Khan will praise Labour leaders and elected mayors in power across Britain, saying: "Labour is in power right now, not just in London, but in Wales too with our First Minister, Carwyn Jones, and in Bristol with our new mayor, Marvin Rees.
"Labour is in power right now in Liverpool, Manchester and Southampton. In Newcastle, Glasgow and Sheffield. In Birmingham, Cambridge, Leeds and Cardiff too. Labour is in power in towns and cities the length and breadth of Britain."
He will call on the party to do everything possible to win mayoral elections for the newly created city regions in Greater Manchester, the West Midlands and the Liverpool region in 2017.