Tim Loughton says he had no intention of voting in party’s leadership contest but wanted to ‘blow the gaff on what a complete farce the whole thing is’

The Conservative former children’s minister Tim Loughton has been caught signing up as a registered supporter of Labour to be eligible to vote in the party’s leadership election.

Under new rules anyone can vote if they pay £3 and sign up as a registered supporter by midday on Wednesday 12 August, prompting concerns that the system could be gamed by Tory troublemakers.

If I’d got a voting paper I was going to tweet myself ripping it up Tim Loughton

Registered supporters must give signed or verbal agreement that they “support the aims and values of the Labour party” and are “not a supporter of any organisation opposed to it”. Figures show 20,000 new members and a further 21,000 registered supporters have signed up since nominations for the leadership closed.

Loughton signed up using his parliamentary email address and home postal address and his application was spotted and blocked in the party’s vetting process.

A Labour spokesperson said that while they were “delighted to hear Tim shares Labour’s aims and values” they would not allow him to vote in the leadership election, stressing that they had a “robust vetting process” to weed out bogus applications.

“We would like to thank him for his generous donation to the Labour party – which we are keeping,” they said. “If he wants to donate more money he is very welcome to do so via our website.”

Loughton, who has been MP for East Worthing and Shoreham since 1997, said he had no intention of actually voting but wanted to “blow the gaff on what a complete farce the whole thing is”.

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“If I’d got a voting paper I was going to tweet myself ripping it up, just to make a point about how ridiculous the whole open exercise is,” he said.

“In the box at the end of the application it asks: ‘What are your reasons for wanting to become a supporter of the Labour party?’ I put: ‘To vote to Jeremy Corbyn and consign Labour to oblivion for a generation’ and then I got a ‘welcome to the Labour party’ email. I wasn’t exactly hiding my intent.”

On Tuesday Harriet Harman emailed every Labour MP with the names of people in their constituency who have signed up as party members or registered as supporters since the election, a sign that she was concerned that the contest could be infiltrated by entryists on the left or right.

The interim Labour leader wants MPs to report any of the new members or registered supporters who are members of other parties or are known troublemakers. Harman wrote: “All concerns raised are being acted on. As a member of parliament your local knowledge and information is important to uphold the integrity of the leadership election.”

Harman told the Guardian last week that a new email was being sent to local constituency Labour parties (CLP), setting out how they could check whether bogus applicants were trying to join as registered supporters.

In the email to Labour MPs Harman wrote: “Nationally there has been, and will continue to be, an extensive and robust process of verification of those who seek a vote either as a new member, affiliated supporter or registered supporter.

“This process checks that they support the Labour party and are on the electoral register. Details of these people are being made available to CLP secretaries and vice chairs (membership) in real time and they have been asked to check there aren’t any people known to them to be members of other parties, or who do not support the Labour party.

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“These people are not eligible to vote. CLPs are asked to report to head office if they have any evidence or suspicions. All concerns raised are being acted on.”

On 15 June, Jeremy Corbyn, the leftwing MP for Islington North, unexpectedly secured – with minutes to spare – the 35 nominations required to stand in the leadership contest, which was triggered by the resignation of Ed Miliband after the party’s general election defeat.

In response, Tory supporters mounted a Twitter campaign with the hashtag #ToriesforCorbyn to elect Corbyn as Labour leader in order to damage the party’s future election chances.

The former Conservative MP Louise Mensch was among a group of Twitter users who changed their profile pictures to include a “Jeremy Corbyn for leader” slogan.