Leftist Jeremy Corbyn is set to be re-elected head of Britain's opposition Labour party on Saturday, emerging victorious from a power struggle with his MPs that has threatened to tear the historic movement apart.

Ahead of the announcement of the result in Liverpool, northwest England, Corbyn issued a plea for unity after what he admitted was a "robust" campaign, and reached out to supporters of his rival Owen Smith.

"Whatever the result, whatever the margin, we all have a duty to unite, cherish and build our movement," the 67-year-old said, promising to take the fight to the centre-right Conservative government.

But Corbyn's victory will be a bitter pill to swallow for the majority of Labour MPs who rebelled against him after the referendum vote for Brexit in June, accusing him of failing to campaign hard enough to keep Britain in the EU.

There are now fears that without a strong Labour opposition, Prime Minister Theresa May's Conservatives may be heading for a "hard Brexit" that would take Britain not only out of the European Union but also out of the European single market.

Many Labour MPs will likely now fall into line but Corbyn's leadership has exposed fundamental differences over the direction of the party.

Corbyn claims his anti-austerity message has energised people who had long given up on mainstream politics, yet his critics say he is unelectable -- while also accusing his supporters of intimidation and of trying to take over the party.

"This is not going anywhere anytime soon. It is entrenched warfare," said Steven Fielding, an expert on Labour at the University of Nottingham.

"There will be no victor in the short term, apart from the Conservatives, of course, who are laughing all the way to the next election," he added.

- 'People are not stupid' -
Corbyn was elected for the first time last year with 59 percent of the vote from party members, and with strong backing from trade unions, but was immediately criticised by MPs who say his left-wing views are outdated.

The divisions widened in the following months and reached breaking point with the EU referendum, which triggered mass resignations from his shadow cabinet and a vote of no confidence in his leadership by 172 of the party's 230 MPs.

But the threat against Corbyn faded into a whimper when lawmakers backed the relatively unknown Smith to challenge for the leadership.

Few commentators believed Smith could win, and the contest appears only to have entrenched divisions between Corbyn and Labour's MPs.

Former Labour foreign secretary David Miliband, who was narrowly beaten to the party leadership by his brother Ed in 2010, punctured any hopes for an amicable resolution with a blistering attack on Corbyn's leadership.

"We have not been further from power since the 1930s," the centrist wrote in The New Statesman magazine this week.

"Nationalisation cannot be the answer to everything; anti-austerity speeches cannot explain everything; corporate taxation cannot pay for everything.

"It doesn't add up. It wouldn't work. People are not stupid."

- 'Miserable, unhappy family' -
Corbyn has been in parliament since 1983 and was a staunch opponent of the party's centrist "New Labour" prime minister Tony Blair, who won three elections.

Before he became leader, Corbyn had never held political office, but is a seasoned campaigner, and appears to have been energised by the challenge to his leadership.

The bearded vegetarian, who set up a mass protest movement against the Iraq War, has a messianic appeal to supporters who have packed his rallies around the country.

But his message does not appear to have resonated with the wider British public, and Labour lags behind May's Conservatives in the opinion polls -- and his personal ratings are dire.

Some Labour lawmakers have said they are thinking about creating a new centre-left party.

However, analysts say most will do everything to avoid this, noting the ill-fated creation of the Social Democratic Party in similar circumstances in the 1980s.

"It looks to me like a miserable, unhappy family trying to coexist," said Tony Travers, a politics expert at the London School of Economics.