By William James

LONDON (Reuters) - Britain's opposition Labour Party ruled on Tuesday that its leader Jeremy Corbyn had the automatic right to stand in a new leadership contest, setting the stage for a struggle between the veteran socialist's supporters in the country and party lawmakers who want to oust him.

The turmoil engulfing 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 changing fast 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 on Wednesday 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 (MPs).

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

But Corbyn's chances of holding on to control won a boost when Labour's National Executive Committee (NEC) ruled that he should automatically appear on the ballot paper rather than have to find 51 lawmakers to nominate him, a task that he would struggle to achieve.

"The NEC has agreed that as the incumbent leader Jeremy Corbyn will go forward onto the ballot without requiring nominations from the Parliamentary Labour Party and the European Parliamentary Labour Party," a party spokesman said in a statement.

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The EU referendum sparked a wave of resignations from Corbyn's policy team, with many saying he didn't campaign hard enough to prevent Brexit, and culminated in the passing of a motion of no-confidence in him by a margin of 172 to 40.

But Corbyn retains strong support among the party's more left-leaning rank-and-file members, meaning that he could hold on to power and prolong the stand-off with MPs.

The NEC ruling could still trigger a legal challenge from opponents who insist he too should have to garner support from 51 lawmakers, but Corbyn played down such a possibility.

"There's been a very long legal discussion this afternoon, there were very well qualified lawyers on hand to advise, so I think we are fine," he told reporters after the NEC decision.

Corbyn said his re-election campaign would be based on tackling inequality and poverty in Britain, adding that he would be reaching out to everyone in the party.

"I am sure Labour MPs will understand that the party has to come together in order to present to the British people the option of a different and better way of doing things," he said.

Eagle said she was pleased the NEC had reached a decision.

"I welcome the contest ahead," she said on Twitter. "And I am determined to win it."

In defying the pressure to resign, Corbyn has cited the overwhelming mandate he won from the party's grassroots members when they elected him leader last September. That has sparked fears that the party could split, as it did in the 1980s, and dilute the center-left influence over Brexit negotiations.

Labour's internal strife has also fueled tensions among its supporters. Police said on Tuesday that Eagle's constituency office in northern England had been vandalized. Corbyn said he and other lawmakers had received death threats.

"It is extremely concerning that Angela Eagle has been the victim of a threatening act and that other MPs are receiving abuse and threats," Corbyn said in a statement.

Eagle blamed the vandalism and threats on Corbyn.

"They are being done in his name and he needs to get control of the people who are supporting him and make certain that this behavior stops and stops now," she told the BBC.

"It is bullying. It has absolutely no place in politics in the UK and it needs to end."

(Additional reporting by Michael Holden and Paul Sandle; editing by Stephen Addison and Gareth Jones)