(REUTERS) – The socialist head of Britain’s opposition Labour Party, Jeremy Corbyn, will discover on Tuesday whether he has the automatic right to defend himself from a leadership challenge – a key moment in the running battle for control of the party.

The turmoil within the 116-year-old party, which governed Britain for 13 years until 2010 under Tony Blair and Gordon Brown, comes as the country’s political landscape is rapidly redrawn following a June 23 vote to leave the European Union.

While the ruling Conservative Party has quickly appointed a new leader, Theresa May, to take over from Prime Minister David Cameron and pilot the ‘Brexit’ process, Labour’s left-wing leadership is locked in a bitter internal power struggle with its more moderate members of parliament.

As Labour looks to define its priorities for the upcoming negotiations, rival MP Angela Eagle has triggered a leadership contest, saying Corbyn has failed to connect with voters and is not capable of winning a national election.

Corbyn has said he will not resign, citing the overwhelming mandate he won from the party’s grassroots members when they elected him leader in September last year. That has sparked fears that the party could split, as it did in the 1980s, and dilute the centre-left influence over Brexit negotiations.

LONDON, ENGLAND – JUNE 27: Supporters of Labour leader Jeremy Corbyn shout and clap during Momentum’s ‘Keep Corbyn’ rally outside the Houses of Parliament on June 27, 2016 in London, England. (Getty)

Later on Tuesday the party’s national executive committee will make a crucial ruling over whether Corbyn’s name is automatically on the ballot paper for the leadership election, or whether he, like his challenger, needs to find 51 lawmakers to back him.

“It would be alien to the concept of natural justice that Jeremy Corbyn is not automatically on the ballot paper,” said Len McCluskey, a Corbyn supporter and head of Unite, the country’s biggest union and Labour’s largest financial backer.

If the ruling goes against Corbyn he could struggle to find enough lawmakers to support his bid, given that Labour members of parliament have already passed a motion of no confidence in him by a margin of 172 to 40.

Corbyn retains strong support among the party’s rank-and-file members, meaning that if he does make the ballot paper he could hold on to power and prolong the stand-off with MPs.

“Let Jeremy go on the paper,” McCluskey said. “Let our members in the Labour Party decide who they want as our leader. What I won’t accept is any sordid little fix which is alien to our traditions.”

Challenger Eagle said a beleaguered Corbyn had been “hiding in his room”, telling the BBC he should “do the right thing” and step down.