Of the other major parties, the Liberal Democrats do not require UK citizenship or previous residency to join Lib Dems Abroad. Overseas members have the same right to vote for leaders and policies as regular members.

Labour, meanwhile, requires all members to either have British citizenship or to have lived in the UK for at least a year.

Since 1985, British citizens who move abroad have remained entitled to vote in general elections for up to 15 years after they move abroad. Research by the University of Sussex published in 2020 showed that UK parties invested relatively little time and effort in mobilising overseas voters.

The Electoral Commission, which regulates UK elections, has no involvement in party leadership elections as these are outside its remit. It does, however, police rules on donations to political parties.

For donations above £500, an individual must be on the British electoral register, but the price of party membership falls below this threshold.

Asked if it would be possible for those ineligible to vote in the UK or foreigners with no ties to Britain to choose the next prime minister, the Conservative Party told The Telegraph that it does not comment on leadership contests.

A Conservatives Abroad spokesman said: “As it says on our website, “Application for membership of the Conservative Party is subject to review by the Party and its constituency associations before final approval.”