May wins battle to be British PM but faces tussle over Brexit

(Repeats story without changes to text)

* Path to premiership cleared by rival dropping out

* May backed "Remain" camp in EU membership referendum

* After referendum she says "Brexit means Brexit"

By Kylie MacLellan

LONDON, July 11 (Reuters) - Theresa May has won the battle to be Britain's prime minister but will face a much tougher struggle once in power -- overseeing her country's divorce from the European Union.

May backed the "Remain" camp during the campaign for Britain's referendum on EU membership on June 23 but has made clear since then that it must now go ahead, saying: "Brexit means Brexit."

But she has also said Britain should not trigger the exit proceedings until London is ready to start negotiations.

Invoking Article 50 of the EU's Lisbon Treaty would begin those talks but lawyers and politicians differ over who has the authority to trigger the clause and whether it is irreversible.

"There should be no decision to invoke Article 50 until the British negotiating strategy is agreed and clear -- which means Article 50 should not be invoked before the end of this year," May, 59, said late last month when she launched her campaign to succeed Prime Minister David Cameron.

That potentially puts May on a collision course with German Chancellor Angela Merkel, who is often portrayed as the EU's most influential politician as leader of its strongest economy.

Merkel said on Monday talks with Britain would "not be easy" and has said she expects London to begin the formal process of leaving as soon as it picks a new prime minister.

May, whose only remaining rival quit the race to replace Cameron on Monday, will be Britain's first woman prime minister since Margaret Thatcher, who governed from 1979 until 1990.

Cameron, who called the vote to appease anti-EU lawmakers in his own party but campaigned for continued membership, said he plans to tender his resignation to Queen Elizabeth on Wednesday. May will take office the same day.

"DIFFICULT"

Article 50 envisages a period of up to two years to negotiate an amicable separation.

Triggering it quickly might weaken London's hand in negotiations on the terms of its break with the EU but any delay could add to uncertainty for investors wondering how the future relationship with the EU will look.

Supporters say she has steely determination, pays attention to detail and focuses on getting on with the job at hand. She has also been described by a Conservative Party grandee, Ken Clarke, as a "bloody difficult woman".

May has made clear she will respect the will of the British people, expressed in the referendum last month.