By William James

BOURNEMOUTH, England (Reuters) - The UK Independence Party picked a new leader on Friday to replace Nigel Farage, a key player in Britain's vote to leave the European Union, with the incoming Diane James pledging to ensure Britain follows through with a full withdrawal from the bloc.

James, a former business analyst and the party's deputy chairwoman, said she would work to ensure that the Conservative government did not negotiate a soft Brexit.

"The threats to the referendum outcome are increasing by the day," she told around 1,000 delegates at the party's annual conference in the southern English coastal town of Bournemouth.

"This is what I want you to believe in and work with me on: Yes to a true, 100 percent, European Union exit."

A member of the European parliament with little public profile, James has the difficult task of succeeding one of the country's most recognizable politicians after winning a ballot of UKIP's 40,000 members with 47 percent of the vote.

She will inherit a party which won 4 million votes in the 2015 national election but is now riven with factional disputes and struggling to redefine itself after achieving its main goal of triggering Britain's exit from the EU.

An ally of Farage defected to the Conservatives on the eve of the conference, saying that Prime Minister Theresa May had delivered key elements of the UKIP manifesto since taking office in July and that droves of UKIP supporters were doing the same.

James, a former analyst with a board-level career in the healthcare sector, said she would ensure the government delivers a Brexit deal that meets the main demands of UKIP voters: more free trade and tighter immigration controls.

"No to unrestricted, or uncontrolled freedom of movement into this country... If they come in, they come in on a fair basis," she said.


James said she wanted to professionalize the party, whose colorful and sometimes controversial members have in recent years provided Britain's tabloid newspapers with a litany of scandals and gaffes.

Former Prime Minister David Cameron once dubbed them "fruitcakes, loonies and closet racists".

"Change is going to have to happen," she said, stating her ambition for UKIP to overtake the Labour Party to become Britain's official opposition.

UKIP came third by share of the vote in the 2015 UK election, but under the first-past-the-post electoral system, which favors the two main parties, won only one of the 650 seats in parliament.

Nevertheless, it remains a potent force in British politics.

Under Farage, who tapped into a powerful anti-establishment mood among voters, the party won the European Parliament elections in Britain in 2014 and played a leading role in persuading Britons to vote to leave the EU.

Its right-wing policy agenda on issues like immigration has traditionally taken votes from the Conservative Party, but at last year's election it made big gains in centre-left Labour's heartlands among disillusioned working class voters.

James chose not to participate in the party's leadership hustings and is little known outside political circles - something she acknowledged in her speech - leaving her with a huge task to maintain the party's momentum.

"I am not Nigel-like, I am not even Nigel-lite. I will never ever pretend to be so," she said.

At a later news conference she said that she would be "nuts" to ignore any advice Farage had to offer but that she would not let him become a "backseat" driver for the party.

(Additional reporting by Tina Bellon; Editing by Michael Holden and Hugh Lawson)