By Elizabeth Piper

LONDON (Reuters) - Interior minister Theresa May led the field on Thursday before a vote to narrow the race to become Britain's prime minister to two candidates, a decision that will help determine the course of divorce talks with the European Union.

May, who has appealed to lawmakers to vote for her wide experience in government as a safe pair of hands, had the support of more than half of the parliamentary party, leading Andrea Leadsom, a junior energy minister, and justice minister Michael Gove ahead of the second stage of voting on Thursday.

But her victory is not guaranteed - tactical voting among Conservative lawmakers on Thursday may skew the result and the final candidates will need to spend weeks trying to win over the party's largely eurosceptic membership, who may think a fresh face would stand a better chance in an election in 2020.

The winner, to be chosen by Sept. 9, will also form a team to guide Britain's future relationship with the European Union after the vote to leave the bloc last month triggered calls in Brussels for the country to start departure talks.

"It looks like May," said an aide to Prime Minister David Cameron, who triggered the contest by saying he would step down on the morning after Britons voted to leave the European Union.

According to ConservativeHome, a website of the party, May had the support of 159 lawmakers of the party's 330 members of parliament who had declared their preference, Leadsom had 48 and Gove 27.

Leadsom got a boost on Thursday when she was backed by former Conservative leader Michael Howard, and as someone who backed the "Leave" campaign she may find support from the wider membership who want Brexit to be delivered quickly.

Speaking to supporters and journalists, she sought to ease concerns among EU migrants that they may have to leave Britain by saying those legally here are welcome to stay. She also said she expected the economy to grow after Brexit.

May has deeper support among lawmakers - winning 165 votes in the first round, but she supported the "Remain" campaign -- albeit with little passion -- and has said Britain should wait before triggering the formal exit procedure, Article 50.

"May is saying she will recognize the Brexit result but ... I think for many there is a real concern given that she was a 'remainer' whether she will deliver," said Anne Marie Morris, who is supporting Leadsom.

"If you are naturally somebody who is a eurosceptic, if you have got any doubt over whether she will deliver on that, then you will go for Andrea."

The campaign to win votes in the ruling party, which was deeply split over Britain's EU referendum last month, has been gripped by horse trading and suggestions the camps backing all three remaining candidates were involved in tactical voting.

Two Conservative Party activists said some of May's supporters were encouraging lawmakers who back the interior minister to vote for Gove, cutting out the increasingly popular Leadsom from the race.

But they also said other lawmakers who are backing May had resolved to "stick it to" Gove for ending Boris Johnson's leadership bid by standing himself in a contest the former London mayor was once favored to win.

"That's the reality of politics," a lawmaker said.

(Additional reporting by William James; Editing by Dominic Evans)