The five hopefuls to lead the post-election revival of Britain's battered Labour party Saturday condemned its struggles with anti-Semitism and vowed to expose the frailties of Prime Minister Boris Johnson.

Their first joint appearance before Labour members in Liverpool comes with the 120-year-old party reeling from its worst election defeat since 1935.

The successor to veteran socialist Jeremy Corbyn will be charged with fighting Johnson's efforts to reshape Britain's economy after he takes it out of the European Union on January 31.

Finance minister Sajid Javid gave an early taste Saturday of the disruptions potentially facing UK companies when the Brexit transition period expires at the end of 2020.

"There will not be (economic) alignment," Javid said in an interview with the Financial Times.

The successor to veteran socialist Jeremy Corbyn will be charged with fighting Johnson's efforts to reshape Britain's economy after he takes it out of the European Union
The successor to veteran socialist Jeremy Corbyn will be charged with fighting Johnson's efforts to reshape Britain's economy after he takes it out of the European Union AFP / Paul ELLIS

"We will not be in the (European) single market and we will not be in the customs union."

The post-Brexit difficulties some economists predict could test the unity of Johnson's majority in parliament and give Labour an issue to rally around ahead of the next scheduled election in 2024.

The candidates on stage in Liverpool -- one man and four women -- attempted to launch their leadership bids by confronting anti-Semitism scandals that hounded the party during Corbyn's rule.

"Jewish people were scared of Labour winning the election. That's deeply serious," centrist candidate Jess Phillips said.

Frontrunner Kier Starmer said it was "quite possible to be critical of Israeli policies without being anti-Semitic.

The 2019 vote was Labour's worst election defeat since 1935
The 2019 vote was Labour's worst election defeat since 1935 AFP / Tolga AKMEN

"If you are anti-Semitic you shouldn't be in the Labour party, it's as simple as that."

And local MP Lisa Nandy -- a longshot who impressed London pundits in her first round of TV interviews -- said she was "ashamed of where our party ended up".

"We gave the green light to anti-Semites," said Nandy. "Never again do I want to be door knocking with members of the party and be called racist".

Labour's sharp shift to the left during Corbyn's four-year rule saw it sign up in December to what was officially dubbed "the most radical (election platform) in modern times".

Candidates to replace Jeremy Corbyn as leader of Britain's main opposition Labour Party
Candidates to replace Jeremy Corbyn as leader of Britain's main opposition Labour Party AFP / Gillian HANDYSIDE

The party was also injected with a new wave of members who embraced socialism and rejected the more moderate "New Labour" approach of former prime minister Tony Blair.

This new membership ultimately decided to take no stance on Brexit at all -- a position aimed at appeasing both the eurosceptic trade unions and EU-backing voters around London.

Post-election studies showed voters pinning Labour's loss on its indecision over Brexit and seeming inability to confront anti-Semitism.

Yet Starmer served as a senior member of Corbyn's shadow cabinet and delivered a keynote address at the party last conference in September.

"Don't trash the last Labour government and don't trash the last four years," he said Saturday.

Second-favourite Rebecca Long-Bailey has been branded a "continuity Corbyn" candidate by the media and is backed by the party's hard left.

She conceded that "we didn't get our messaging right" and "were not dealing with (anti-Semitism) effectively".

But both she and Starmer called for party unity and castigated the last nine years of Conservative rule.

"You can really see when a government has washed its hands of the people," said Long-Bailey.

Labour's foreign policy spokeswoman Emily Thornberry -- a veteran politician who is seen as a longshot -- said Johnson had a lax grasp of policies and should be "held to account".

"I am a girly swot, I would quote things at him. He would look confused," she said of her debates with Johnson when he was foreign secretary in 2016-2018.

Phillips adopted the same line of attack.

"I have managed to take on Boris Johnson from the back benches and have silenced him," said Phillips.

"Johnson would be terrified at facing me."

A poll of Labour members released ahead of the debate showed Starmer beating Long-Bailey in the final round leadership race of ballots by 63 percent to 37 percent.

Phillips was trailing distant third with 11 weeks of campaigning to go.