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".
His comments, which are likely to be seen as a coded criticism of Jeremy Corbyn's left-wing policy agenda, come after a string of senior Labour figures including former cabinet minister Lord Blunkett warned the leader's approach was rendering the party "unelectable" and threatening its future survival.
A clutch of polls released during the four-day conference have put the party's support as low as 26%, with one pollster calculating it could be reduced to fewer than 200 MPs in the House of Commons for the first time since 1935.
But shadow chancellor John McDonnell brushed off the gloomy predictions, insisting that Labour is a "government in waiting" and blaming its current slump in the polls on the bruising leadership contest which ended with Mr Corbyn's decisive defeat of challenger Owen Smith on Saturday.
Mr Khan - whose election as London mayor gave him the biggest personal mandate of any politician in UK history, with more than 1.1 million votes - 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.
"It's only with Labour in power that we can create a fairer, more equal and more just Britain," he will warn.
"Labour out of power will never, ever be good enough ... The people who need us the most are those who suffer the most when Labour is not in power.
"It's time to put Labour back in power. It's time for a Labour government. A Labour prime minister in Downing Street. A Labour cabinet. Labour values put into action."
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, saying: "We have to start by winning the mayoral elections next year, here in Liverpool, along the M62 in Manchester and down the M6 in Birmingham. Let's ensure Labour is in power in every great city in Britain."
Labour mayors in office "can demonstrate that we can make a real difference to people's lives" and that "with Labour in power we can prove that we are ready for government", he is expected to say.
Meanwhile, shadow health secretary Diane Abbott will tell the conference that Labour needs to "come to the rescue" of the NHS after being "pushed to the brink" by the Conservative Government.
Ms Abbott is expected to say: "Labour will restore the NHS to a properly functioning service after years of Tory cuts, damaging reorganisations and privatisations.
"As the party which founded the NHS, Labour will once again be obliged to come to its rescue after a Tory Government has once more pushed it to the brink. Labour will restore it."
In her keynote speech, Ms Abbott will promise to focus investment on public health and preventative measures and to prioritise mental health services, particularly for young people.
She will promise to "renationalise" the NHS by restoring the accountability of the health secretary which was removed by the controversial 2012 Health and Social Care Act.
And she will say that Labour would end marketisation and remove private provision where it is "ripping off" the NHS, as well as setting up a new unit to cut out waste caused by private finance initiative (PFI) contracts and agency staffing.
Meanwhile, deputy leader Tom Watson will launch an independent commission into the future of work, to look into the challenges of new technology and automation, as well as what he referred to as "the dark side of the gig economy".