The KGB could have stooges ready to tip the balance of the Tory leadership race if it goes to an online vote, a top cyber expert has warned.

Peter Ryan, a professor of applied security at the University of Luxembourg, told the PA news agency little is known about the Conservative Party membership.

Figures vary for how many members there are, with 180,000-200,000 usually given as estimates.

The party’s website has a membership option called “Conservatives Abroad”, which says “anyone living anywhere in the world is welcome to join Conservatives Abroad from just £25 per year”.

It adds: “As an overseas member, you are entitled to all the benefits of party membership, including participation in the Conservative Policy Forum, attendance at party conferences and a vote in the election of the party leader.”

While many overseas Tory voters may be legitimate UK citizens living overseas, others could be KGB stooges, Prof Ryan said.

“They are not prepared to say anything about their membership,” he said.

“It seems to be easy to register as a non-UK voter.

“We don’t know that much about the electorate that are putting in place the leader of a G7 country. There could be a lot of people overseas that are not even UK citizens.

“For all we know, the KGB could have signed up a significant number of stooges.

“The margin last time was low – it would not take much to swing it.”

Online voting is “always vulnerable” to hacking, “especially by state attackers”, Prof Ryan added.

In the previous Tory leadership race, party members were initially told they could vote by post and amend their decision online until the system was reformed in early August.

The National Cyber Security Centre (NCSC), part of GCHQ, said the process was vulnerable to interference, forcing the party to delay sending out ballot slips.

Those opting to cast their vote online had to submit a single-use code and answer security questions.

Commenting on the prospects of another online vote, the NCSC said it continues to advise the Conservative Party.

“Defending UK democratic and electoral processes is a priority for the NCSC and we work closely with all parliamentary political parties, local authorities and MPs to provide cyber security guidance and support,” a spokesman said.

“As the UK’s national technical authority for cyber security, we continue to provide advice to the Conservative Party, including on security considerations for online leadership voting.”

The Conservative Party was contacted for comment.