By Kylie MacLellan and Alistair Smout

LONDON (Reuters) - Britain's opposition Labour Party is set for a bitter battle over its future after leader Jeremy Corbyn said he expected to be on the ballot automatically for a leadership contest and would fight any attempt to prevent him entering the race.

Labour has been thrown into crisis since Britain's vote last month to leave the European Union, with Corbyn refusing to step down despite mass resignations from his policy team and a no-confidence motion passed by an overwhelming majority of his party's lawmakers.

One of them, Angela Eagle, has said she will formally launch a bid on Monday to replace Corbyn as party leader, saying he has failed to lead Labour parliamentarians as an organized and effective force to hold the government to account.

Eagle said an election, due in 2020, could come early and her party needed to be ready.

"(Corbyn) doesn't connect with Labour voters, he doesn't connect enough to win an election ... and I really think now the time is for him to consider his position," she said.

Corbyn told the BBC: "I'm expecting to be on the ballot paper because the rules of the party indicate that the existing leader, if challenged, should be on the ballot paper anyway." He said legal advice he had received backed this view.

Eagle said she had the 51 nominations by lawmakers required to mount a challenge and that while she believed it was party precedent that Corbyn would need the same level of support from colleagues to take part in the leadership ballot, it would be up to the party's national executive committee to decide.

Some Labour lawmakers fear Corbyn, who spent three decades on the hard-left fringe of the party before a surprise wave of grassroots enthusiasm swept him to victory in a leadership contest last September, will be voted back in by the party membership if he makes it on to the ballot.

"I am not contemplating losing this contest, I am in it because I want to win it for the best interests of a strong Labour Party," Eagle told ITV.

"This is a battle for a healed and a united Labour Party."

Corbyn said he was disappointed with Eagle's bid to unseat him and would challenge any decision by the national committee to block him from standing again.

"I would also just ask anyone in the party to just think for a moment: is it really right that the members of the party should be denied a decision, a discussion, a choice in this?" he said. "It is a democratic party not a dictatorship. I was elected by a very large majority of members and supporters."

RIGHT PERSON TO LEAD

The ruling Conservatives are also in the midst of a leadership contest after Prime Minister David Cameron, who campaigned for Britain to stay in the EU, announced on the morning after the referendum he would resign.

The Conservative's 150,000 members will vote by Sept. 9 on which of two candidates, interior minister Theresa May or junior energy minister Andrea Leadsom, should replace him as party leader and prime minister.

While May received the support of 199 Conservative lawmakers, compared to 84 for Leadsom, some expect Brexit-supporting Leadsom could attract stronger support among the party's more Eurosceptic membership.

Junior minister Priti Patel, who campaigned for Brexit but is now supporting May for the leadership, said the Conservative Party risked mirroring Labour's split between lawmakers and grassroots members.

"To govern we have got to be able to carry the support of members of parliament. That's incredibly important. I don't need to give a re-run of what's happening with Labour right now," she told the Sunday Telegraph.

"We could end up in that situation and then it becomes very difficult to govern and deliver the program for Leave."

(Editing by Janet Lawrence)