By William James
LONDON (Reuters) - The contest to decide the new leader of Britain's opposition Labour Party may have been infiltrated by several thousand supporters of Prime Minister David Cameron's Conservatives, one of the candidates vying for control of the party said on Friday.
In a letter to Labour's general secretary, candidate Andy Burnham said he was worried that the contest's outcome could be subject to legal challenge if it had been infiltrated.
The choice of a new Labour leader after a bruising election defeat in May has become an ideological battle between hard-left Jeremy Corbyn and three other more-centrist candidates - Burnham, Yvette Cooper and Liz Kendall. Who wins will define how the party opposes Cameron's government and contests a 2020 election.
Corbyn is a long-time Labour outsider who is campaigning to return the party to its socialist roots, including re-nationalising railways and energy networks. A poll earlier this month forecast he would get 53 percent of the first-round vote, enough to win outright.
That prospect delights many Conservatives. They believe a Corbyn victory would effectively bequeath them the votes of centrist Britons and improve their chances of holding on to power in 2020.
The right-wing Daily Telegraph even urged Conservatives to sign up as Labour supporters and vote for Corbyn. One Conservative lawmaker has already said publicly that he tried to vote in the contest but was rejected.
Changes to the rules of the leadership contest make it easy for supporters to sign up and vote for a three-pound ($4.70) fee. That has raised fears the contest may be open to manipulation.
"There ... remain concerns about potential Tory (Conservative Party) infiltration on a large scale," said Burnham, highlighting the large number of new sign-ups.
"The 121,000 registered supporters could include several thousand Tory infiltrators, as well as supporters of other parties seeking to have a vote in the election."
A Labour spokeswoman said it had a robust vetting system in place to make sure that those signing up to vote were genuine Labour supporters. Last week, Labour said it had disbarred 1,200 people.
The party said it had no plans to delay the voting.
($1 = 0.6382 pounds)