Conservative members are facing delays in receiving their postal ballots to vote for who they want to be the party’s next leader due to security fears.

In a letter sent to Tory members seen by Sky News, the Conservative Party’s head of membership confirmed postal ballots will arrive “a little later than we originally said” as “we have taken some time to add some additional security” to the process.

Members were previously due to receive their postal votes this week.

However, the email sent to members says they should receive their ballot by Thursday 11 August.

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The correspondence adds that voting more than once in the ongoing leadership contest will be treated as “an offence” and warns that any member who is found to have voted multiple times will “have their party membership withdrawn”.

It adds that Tory members can either vote by post or online.

But the Conservative Party has confirmed that the security fears have forced it to abandon plans to allow members to change their vote for the next leader later in the contest.

Allies of leadership hopeful Liz Truss were believed to have been concerned the original rule allowing Tory members to change their vote in the contest would work to her rival Rishi Sunak’s advantage.

The rules of the leadership contest, set by the 1922 Committee of backbench MPs and the Conservative Party board, state members should only vote once but if a “duplicate” vote is recorded, the second one will be counted.

CCHQ described it as a slight delay and were unable to give any further guidance.

A Tory Party spokesperson 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.”

Earlier on Tuesday, a new poll suggested Ms Truss has extended her lead over Mr Sunak in the leadership race.

It comes as Mr Sunak battles to make up ground during what is a key week in the contest for the keys to Number 10.

The latest YouGov poll of Conservative members for The Times newspaper shows Ms Truss’s lead has stretched to 34 points in the Tory leadership race, with 60% of party members now saying they will vote for the foreign secretary to succeed Boris Johnson as prime minister.

This is compared to just 26% for former chancellor Mr Sunak.

The poll, carried out over the last five days, shows Ms Truss is now ahead of Mr Sunak among all age groups, across different parts of the country and with men and women.

The only category where he beats Ms Truss is among Tory Remain supporters.

The YouGov survey published on Tuesday also found almost nine in 10 Conservative members have now made up their minds how they will vote ahead of ballot papers going out this week.

But it will concern both potential leaders that more than 50% of party members believed whoever was elected to succeed Mr Johnson would lose the party its majority at the next election.

Just 19% of members thought Mr Sunak could lead the Tories to victory, while 39% thought Ms Truss could see off a challenge from Labour.

Voting will close on 2 September with the winner expected to be announced on 5 September.

As the leadership race continues to heat up, Mr Sunak and Ms Truss will once again face the cameras this week on Sky News.

Taking place on Thursday 4 August at 8pm at Sky Studios in west London, The Battle for Number 10 will see the candidates take part in back-to-back questioning from the live studio audience made up of Conservative Party members who remain largely undecided on who to vote for.

This will then be followed by an in-depth interview with Kay Burley.

The programme will be broadcast live for 90 minutes and for free on Sky News channel 501, on Freeview 233, on Sky Showcase channel 106, and across Sky News’ digital channels.