Ballot papers for the Conservative leadership election have been delayed in being sent out over security fears.
The party said it had changed its plans for the contest, which will decide the next prime minister, after consulting with security agency GCHQ.
The Daily Telegraph reported that GCHQ had warned that hackers could change people’s votes.
Tory members deciding between Liz Truss and Rishi Sunak should get their ballot packs this week, the party said.
Around 160,000 Conservative members are due to elect Boris Johnson’s successor, with the winner announced on 5 September.
It was initially planned that the voting process would have allowed members to choose whether to vote by post or online and then, if they changed their minds, use the alternative method to cancel out their previous vote.
But after advice from the National Cyber Security Centre, part of GCHQ, the Conservative Party has decided to make changes to “enhance security around the ballot process”.
In an email sent to members, seen by the BBC, the Conservative Party said that their voting pack was on its way but would “arrive with you a little later than we originally said… because we have taken some time to add some additional security to our ballot process”.
The email says that, once the ballot company receives a postal vote, they will deactivate the member’s online codes, “reducing the risk of any fraud.”
The Telegraph said it understood fears were raised that scores of votes could have been changed by nefarious actors, although there was no specific threat from a hostile state.
A Conservative spokesperson said: “We have consulted with the NCSC [part of GCHQ] throughout this process and have decided to enhance security around the ballot process. Eligible members will start receiving ballot packs this week.”
A National Cyber Security Centre spokesperson 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.”
In the email to members the Conservative Party said members who had not received their ballot pack by 11 August should contact the party.
The delay comes as a YouGov poll, reported in the Times, found Foreign Secretary Ms Truss had the support of 60% of Tory members, while ex-chancellor Mr Sunak was backed by 26%.
The pair have been meeting in a series of clashes on television and in front of voters and they will take part in a hustings in Wales later.
Much of their debates has focused on tax policy, with Ms Truss vowing to scrap a planned rise in National Insurance, while Mr Sunak has said he would cut taxes “responsibly” including cutting the basic rate of income tax from 20% to 16% by the end of the next parliament.
It has been a mixed few days on the campaign trail for Ms Truss after she received backing from former rival Penny Mordaunt but then was forced to scrap plans to link public sector pay to local living costs following a backlash. She said her plan had been “misrepresented”.