SIX thousand non-Labour members in Scotland have signed up to vote in the party's leadership elections, it has emerged, as acting UK leader Harriet Harman raised fears of possible infiltration by political opponents.

Ms Harman has written to all Labour MPs asking them to look out for suspicious names appearing on lists of new party supporters after 50,000 signed up across the UK.

The move came as the party blocked an attempt by Conservative MP Tim Loughton to register to vote in the UK leadership election.

The MP said he had no intention of using his vote but wanted to embarrass Labour, by exposing its lack of controls, had his application been accepted.

While mischievous Tories have discussed registering to back left-winger Jeremy Corbyn in the the belief he would make the party unelectable, Labour members have voiced greater concern over "entryism" by hard left activists.

For the first time, Labour will use a one-person-one-vote system in elections for the Scottish party leader later this month and UK leader in September.

In addition to party members, two categories of non-member supporter will have a vote.

"Affiliated supporters" sign up through an affiliated organisation such as trade unions.

"Registered supporters" pay a £3 fee and make an online declaration saying they "support the aims and values of the Labour Party" and do not support its opponents.

The system replaces a complex electoral college which gave parliamentarians, grassroots members and affiliated supporters each a third of the overall vote.

Insiders played down fears over entryism in the Scottish Labour leadership election, which is being contested by two centrist candidates, Kezia Dugdale and Ken Macintosh.

However, it is thought the number of non-member supporters from trade unions could boost the chances of Cowdenbeath MSP Alex Rowley in the deputy leadership race.

It is understood around 6000 affiliated and registered supporters have signed up in Scotland, boosting the 15,500 membership electorate by nearly 40 per cent.

They will be entitled to vote in both the Scottish and UK leadership elections.

Across the UK, as many as 50,000 non-members could take part in the contest to succeed Ed Miliband, who resigned in the wake of May's General Election defeat.

The LabourList website reported that the last meeting of Labour’s ruling National Executive Committee on July 21 was told that since the electionb some 68,000 new members had joined the party together with 21,000 registered supporters and 28,500 affiliated supporters.

Ms Harman wrote to all Labour MPs asking them to use their "local knowledge and information" to "uphold the integrity of the leadership election".

A senior party source said many staff had been committed to “sifting” applications from people wanting to become registered members to ensure there was nothing untoward going on.

"We’re doing everything we can to be on top of this," he said.

Labour MP Fabian Hamilton made clear he had seen "no evidence" of other parties and groups trying to hijack the contest.

But John Ferrett, Leader of the Labour Group on Portsmouth City Council, said the names of a local Conservative activist and a member of the Trade Unionist and Socialist Coalition had appeared on a list he had seen.

A Scottish Labour spokesman said: "We have thousands of registered supporters signing up in Scotland and they are required to confirm support for the aims and values of the Labour Party.

"Any evidence otherwise will result in them being rejected and their leadership voting rights removed."

The result of the Scottish Labour leadership election will be announced on August 15 and the UK contest on September 12.