Labour leadership hopeful Sir Keir Starmer has called for unity and said “factionalism has to go” if the party is to recover from its election defeat.
Speaking at his campaign launch in Manchester, he said: “We are not going to trash the last Labour government… nor are we going to trash the last four years [under Jeremy Corbyn]”.
He has also vowed to end anti-Semitism in the Labour Party.
Sir Keir is one of six candidates running to replace Mr Corbyn as leader.
The shadow Brexit secretary has won the backing of the UK’s largest trade union, Unison.
However, on Saturday the grassroots group Momentum said it will ballot its members on backing Rebecca Long Bailey in the contest.
During his speech on Saturday, the MP for Holborn and St Pancras said: “We can’t fight the Tories if we are fighting each other. Factionalism has to go.”
He criticised Prime Minister Boris Johnson, describing him as a “man of no principles and no moral compass, who will go anywhere to stay in power”.
However, Sir Keir said he would not “trash” the Labour governments of Tony Blair or Gordon Brown, or the previous leadership of Mr Corbyn. He said there had been “many important moves” made.
“Jeremy Corbyn was right to make us the party to fight austerity,” Sir Keir said. “We build on that, we don’t trash it going forward.”
He said Labour should treat the 2017 manifesto as its foundation going forward, saying the next manifesto must “give hope to people that the next 20 years can be better with a Labour government”.
Speaking to the BBC after the speech, he said: “I think what we need to do is make a radical and relevant case to [voters] for change. They need to know it’s going to work and trust us to implement it.
“I’m absolutely committed to the fundamental change needed to deal with the rank inequality in this country.”
BBC political correspondent Nick Eardley called Sir Keir “the man to beat” in the contest and said the leadership hopeful was “not shying away from being radical”.
He added: “But it’s interesting that he said 2017’s manifesto should be a foundation – that was a lot less radical than the 2019 manifesto, which many in the party believed offered far too much far too quickly.”
Earlier, Sir Keir told BBC Breakfast he would personally take charge of the fight against anti-Semitism in the Labour Party.
“If you’re anti-Semitic you should not be in the Labour Party. It is not complicated,” he said.
Sir Keir insisted that anyone who is anti-Semitic should be “chucked out” and said he would take “personal responsibility” for the issue.
Sir Keir was the first of the six Labour leadership contenders to secure the 22 nominations required to progress to the next stage of the contest.
Shadow business secretary Mrs Long Bailey and backbenchers Lisa Nandy and Jess Phillips have also received the required support.
Shadow foreign secretary Emily Thornberry, who currently has 10 nominations, and Clive Lewis, with four, are seeking more support.
Mrs Long Bailey also addressed anti-Semitism at a Labour event in Staffordshire on Saturday, saying “we’ve got to make sure this never happens again”.
She added: “Voters didn’t trust that we were united within our party. Our voters expect us to be united and professional – and yes, we are passionate about what we believe in because it matters so much.
“But that passion must never spill over into abuse, wherever it is coming from.”
A new leader and deputy leader will be announced on 4 April.
Momentum has said it will ballot its members early next week on its recommendation to back Mrs Long Bailey and Angela Rayner for leader and deputy leader respectively.
Following a meeting of the organisation’s steering group, it issued a statement saying Mrs Long Bailey was the “only viable candidate” able to build on the party’s “socialist agenda”.
“We need a new generation of left-wing MPs to lead our party and build on Labour’s popular policy agenda,” a spokesman said.
But Laura Parker, Momentum’s national co-ordinator, said the organisation’s leadership should not have “decided in advance” of the ballot which candidates to support.
“Members should be able to choose from all Leader & Deputy candidates,” she said on Twitter.
Momentum also said it was recommending support for Ms Rayner as deputy, saying the pair could “work well together” and “unite the party against the Conservatives”.