Further evidence emerged today that Britain’s main opposition Labour Party is being infiltrated by the far-left after insiders released the names of senior members of hard-line parties who had joined in order to vote in the leadership election.

Labour began looking for a new leader after Ed Miliband resigned following the party’s defeat in May’s General Election. The contest to replace him has been thrown into turmoil, however, after hard-left MP Jeremy Corbyn unexpectedly won enough support to stand and has subsequently seen a surge in support.

There are widespread fears that members of the far-left in Britain are now joining the party en masse in order to vote for Corbyn, who had been dismissed as an inexperienced outsider but is now favourite to win.

Although Labour claims to be vetting new members, The Times has received the names of various hard-liners who have signed up and not been kicked out.

Eleven people who stood as candidates for the Trade Union and Socialist Coalition (TUSC) – which incorporates the Socialist Party and Socialist Workers Party – have joined the Labour party unnoticed, as have 18 current and former members of the national council of Left Unity, which is backed by the Communist Party.

The 29 names were leaked by insiders concerned that the party’s vetting process is not strong enough to keep out all the hard-left entryists joining the party. People listed include Mark Serwotka of the Public and Commercial Services Union, who has previously harshly criticised Labour for not being sufficiently left-wing. Another person named is Liz Davies, who was on Left Unity’s governing body until this summer.

The race is now on to stop a far-left takeover of Britain’s second largest party as ballot papers for its leadership election are sent out from next week onwards. The party’s website says that 42,250 people have joined since the election and insiders fear party officials do not have the capacity to check all the names before ballot papers are posted.

Harriet Harman, acting party leader, has written to MPs urging vigilance in checking new members in their area.

The revelations follow yesterday’s news that the party had already been forced to stop over 100 Green Party candidates from joining and voting in the leadership election.