Britain's opposition Labour leader Jeremy Corbyn will use a keynote conference speech Wednesday to present himself as the man to lead Britain but faces a daunting task uniting his rebellious MPs over the divisive Brexit question.

The veteran leftist is expected to dodge direct commitments on the toxic issue, and instead advance a radical domestic agenda built around themes of social justice.

He hopes it will appeal to voters who could be heading to the polls if Prime Minister Theresa May's government collapses under the pressure of Brexit before Britain is set to leave the EU in March.

Corbyn is due to use his speech in Liverpool, north west England, to promise an expansion of free childcare for poor households, saying that "driving up standards of childcare will make that vital difference for millions of our children".

But Brexit has loomed large over the four-day party event, causing ructions that the leader will be keen to avoid on Wednesday.

"I think Corbyn's speech will be built around the theme of government-in-waiting, 'this lot have failed on everything from Brexit to the NHS, they've let the country down, it's time for a change'," Anand Menon, Professor of European Politics and Foreign Affairs at King's College London, told AFP.

With the Brexit negotiations entering their final phase, a conference spat between two senior MPs gave Corbyn a taste of the showdown looming within the party.

Shadow finance minister John McDonnell, a key Corbyn ally, and Brexit secretary Keir Starmer both publicly pressured Corbyn on whether or not to rule out another referendum on Britain's EU membership.

Election calls 

Party delegates voted on Tuesday to support "all options remaining on the table" on Brexit, including campaigning for a second vote if May is unable to get a final deal through parliament.

But veteran leftist McDonnell went off message on Monday, saying the option of staying in the European Union should not be on any ballot paper.

The pro-EU Starmer hit back, receiving rapturous applause from members when he also veered off script to insist that remaining in the bloc was still on the table, exposing the power struggle behind the veneer of party unity.

Corbyn, a long-time eurosceptic, must find a way to reconcile his pro-EU MPs and members with the party's working class voter base, who broadly support Brexit, if he is to capitalise on May's vulnerability at the ballot box.

The leader kept his options open on Tuesday, telling the BBC: "We haven't said there's going to be anything yet.

"What we've said (is that) all options must be considered if and when this government collapses or its negotiations collapse."

For now, the party can unify around attempts to bring down the government, the first step of which will be voting to reject any deal May strikes with the EU, which Starmer said was "increasingly likely".

May would then need the support of almost all of her MPs to get it through parliament, an unlikely scenario given her own party's divisions.

Labour has also vowed to block a no-deal Brexit, and suggested a general election should be called in such a scenario, although pro-EU David Lammy doubted it would happen.

"Most people here want a general election and want Jeremy Corbyn running the country, in the end that will be down to getting a two-thirds majority in the House of Commons, my own view is that's not likely to happen," he told AFP.

Another stumbling block in the way of Corbyn's prime ministerial ambitions is the accusation that he has failed to tackle anti-Semitism within the party -- an issue that has dogged him since he became leader in 2015.

"It will be good to have some strong statements from the platform, saying 'ok, we got this wrong'," said Menon. "They can't pretend it hasn't happened."