The Conservative Party has delayed sending out ballot papers for the leadership election after security concerns have arisen.

The party made changes to its process following the advice of the National Cyber Security Centre (NCSC), part of GCHQ, following warnings that hackers could potentially change members’ votes.

Under the original system, members would be able to vote but change their decision while the ballot remained open.

However, the new process will see a unique code be provided which will only allow one, unchangeable vote.

The ballots were due to be sent out from Monday, but could now arrive as late as Thursday, August 11.

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.”

The Daily Telegraph said it understood there was no specific threat from a hostile state and concerns were around the vulnerability of the voting process.

The party has been forced to abandon the plans to let members change their vote if they alter their decision about which candidate to back during the election process, the newspaper reported.

A message sent to 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The message to members said “it is an offence to vote more than once” – with the threat of withdrawn membership for anyone found to have done so.

Former party treasurer Lord Cruddas of Shoreditch, who has been campaigning to keep Boris Johnson in office, said the leadership contest should be suspended as a result of the 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.