Jeremy Corbyn is expected to address Labour MPs after being warned by London mayor Sadiq Khan that he must appeal to voters beyond the party's core support.
The Labour leader has been heavily criticised by some of his own MPs following a mixed set of election results which saw the party relegated to third place in Scotland but perform better than many had feared in England.
In one of the highlights of the elections for Labour, Mr Khan won the mayoral race in the capital, but used a series of media appearances over the weekend to issue a warning about the party's direction.
Mr Corbyn was notable absent from the mayor's signing-in ceremony, but the pair are expected to hold talks today.
Asked on BBC One's The Andrew Marr Show whether he owed some of his election victory to Mr Corbyn, Mr Khan replied: "Success has many parents and I think what's important is the victory on Thursday was a victory for London because what it showed was London chose hope over fear and unity over division.
"My point is very simple, we've got to stop talking about ourselves and start talking to citizens about the issues that matter to them."
Mr Khan said that "it should never be about 'picking sides'" - strikingly similar language to the "Elections are about taking sides" slogan used on the party's posters.
Asked if he was directly attacking Mr Corbyn's campaign strategy, Mr Khan said: "My point is this. I want a big tent, you know, whether you're a Conservative trying to be the Mayor of London, or a Labour Party trying to form the next government, we've got to speak to everyone.
"There's no point in us just speaking to Labour voters, our core vote."
Mr Corbyn, who travelled to Bristol to celebrate with that city's new mayor Marvin Rees rather than attend Mr Khan's ceremonial event, has insisted the pair are "getting on fine".
He is expected to address a meeting of the Parliamentary Labour Party in Westminster on Monday evening.
Shadow chancellor John McDonnell vowed that he and Mr Corbyn would unite the party, telling BBC Radio 4's Westminster Hour: "When Jeremy was elected he invited everybody who was in the existing shadow cabinet to come and play a role, including people like Caroline Flint and others, and he created that big tent and some of them have just excluded themselves from offering their services to the party and that's up to them.
"That's fair enough. Of course people can express their dissent and we can have that policy debate and then we'll arrive at a democratic decision and then we expect people to unite in the interests of the party and the country overall.
"What we don't expect is people going to the media two days before polling to say there is going to be a coup against the leadership. That is not the loyalty we'd expect from any element of the party.
"However we will move on from that. We'll unite the whole party. Every one of those dissenters will be invited in to see Jeremy, they will offered roles in the campaign in the future and in our administration.
"Everyone has got the opportunity to serve and we hope then we can unite the party and move on and campaign now into the referendum campaign."
Former leadership contender Liz Kendall echoed Mr Khan's criticism, telling The Westminster Hour: "It is stating the obvious that Jeremy has got huge support amongst the membership. The challenge for us now is to set out a vision for the country that reaches out beyond our core base."
She added there was a "huge longing for a genuine alternative" to the Tories "and that's where we have got to be".