Tory chiefs have delayed sending out ballot papers for the leadership election amid hacking fears.
The party has overhauled the system for electing Boris Johnson ’s successor following advice from the National Cyber Security Centre - part of eavesdropping post GCHQ - that hackers could change members’ votes.
Under the original process, activists would be able to vote but change their decision while the ballot remained open.
But now a unique code will be provided which will only allow one, unchangeable vote.
The ballots had been due to be sent out from Monday this week but could now arrive as late as next Thursday.
A Conservative spokesman said: “We have consulted with the NCSC throughout this process and have decided to enhance security around the ballot process.
“Eligible members will start receiving ballot packs this week.”
An NCSC spokesman said: “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.
“As you would expect from the UK’s national cyber security authority, we provided advice to the Conservative Party on security considerations for online leadership voting.”
Reports suggest there was no specific threat from a hostile state and concerns were around the vulnerability of the voting process.
But Russia has repeatedly been accused of interfering in overseas votes, including the 2016 Brexit referendum, the 2016 US Presidential ballot and the 2019 UK general election.
A message sent to Conservative members said if they wish to vote online they will have to enter their single-use code and fill in security questions.
If they choose to vote by post, their online code will be deactivated once the ballot is received, “reducing the risk of any fraud”.
The message to members said “it is an offence to vote more than once” - with the threat of being kicked out of the party for anyone found to have done so.
Former party treasurer Lord Cruddas of Shoreditch, who has been campaigning to keep Mr Johnson in No10, said the leadership contest should be suspended because of cyber security concerns.
In a letter to the party’s board, he said members should then be given a vote on whether to accept Mr Johnson’s resignation.
“If the members vote to keep Boris then there is no need for a leadership campaign and no more cyber security threats,” he said.
The decision to shake up the voting process came as bookies’ overwhelming favourite for the Tory crown, Liz Truss, faced Tory members for the first time after making a “full fat U-turn” over public sector pay.
The Foreign Secretary was forced to ditch plans to slash pay of public sector workers depending on where they live.
She and rival Rishi Sunak are taking part in the latest hustings for members in Cardiff tonight.
Ms Truss’s campaign enjoyed a boost as a YouGov poll for The Times found 60% of party members planned to vote for the Foreign Secretary, with 26% set to back former Chancellor Mr Sunak.
Asked if she was looking forward to being PM, Ms Truss said: “I am not taking anything for granted. There is still a long way to go in this leadership election.”
The next hustings take place on Friday night in Eastbourne, East Sussex.
