Jeremy Corbyn will admit Labour is not on course to win the 2020 general election but demand an end to public sniping by critics of his leadership.
The Labour leader will directly confront unrest fuelled by last week's election results when he addresses the weekly meeting of his MPs and peers at Westminster.
Senior colleagues have clashed openly on social media amid an increasingly fraught atmosphere within the Parliamentary Labour Party - though there appears to be no prospect of any imminent challenge.
Corbyn will concede that Thursday's results - when he became the first opposition leader for 50 years to lose council seats in his first local elections and saw the party hammered in Scotland and fall back in Wales - were "mixed".
He will also acknowledge the need to secure the backing of voters across the political spectrum.
But he will insist the party's electoral recovery after the 2015 general election loss "has begun in earnest" and make a firm appeal for the party to unite behind him and focus attention on fighting the Conservatives.
"I don't expect, or even want, blind loyalty, but members and supporters expect us all to focus on taking on the Tories - and for our debates to be focused on policy, not personality," he is due to tell them.
"Members also tell me that they don't think Labour MPs should be parading on the media to give a running commentary on our party. If we are on the media we are there to give our verdict on this failed and divisive government, not on each other.
"We need, if not across-the-board unity, then at least respect for each other - and to turn our fire on this Tory government, and its forced academisation, tax and disability cuts policies in utter disarray.
"It's been said in the past few days we need to stop talking about ourselves and engaging with the concerns and priorities of the wider public. I suggest we all follow that advice."
Before the PLP meeting, Corbyn held talks with the new London Mayor - who was widely seen to have distanced himself from the leader's left-wing agenda during the campaign.
The party leader was a notable absentee from the City Hall signing-in ceremony for Sadiq Khan, who used a series of media appearances over the weekend to issue warnings about the party's direction.
A spokesman for Khan said the pair's 30-minute meeting was "friendly and businesslike" and centred on a number of "matters of mutual interest" including housing, transport and Europe.
Khan was due to tell the behind-closed-doors PLP meeting the party "cannot afford to miss any open goals".
He was repeating his warning against taking an "us and them" approach - a clear swipe at the party's election poster telling voters it was "about taking sides".
And he said there was "no such thing as glorious defeat".
"The incompetence and internal divisions that we've seen from the Government in recent months is reminiscent of the Major years in the 1990s - a Government in decline," he was expected to say.
"We cannot afford to miss any open goals. Just like we did then, Labour has a responsibility to hold the Government to account for its failures and show we are a credible government-in-waiting.
"We are not there yet, but I know with the right approach, Labour can still win in 2020.
"If this election tells us anything, it is that it is so important that Labour wins elections. When we win we can change lives for the better. There is no such thing as glorious defeat.
"I learnt a lot during this campaign - about myself, about the city I love and about elections.
"Two things stand out as lessons for the Labour Party. First, we only win when we talk about the issues that people care about most. And second, we must be a big tent that appeals to everyone in our country, regardless of their background. We lose when we take an 'us and them' approach.
"I intend to prove through action rather than words that Labour can be trusted to govern and that in power, we can change lives for the better."