Voting has been delayed in the Conservative leadership contest because of fears that cyber hackers could interfere with the race to be Britain's next prime minister.
Party members were told that their ballots would be arriving late after a last-ditch change in which the option to amend an online vote will be removed.
The delay in voting came after discussions between British security officials and the Conservative Party, which said it had "taken some time to add some additional security to our ballot process, which has delayed us slightly".
The Daily Telegraph reported that spy agency GCHQ had raised fears of cyber hackers changing people's ballots. It was not thought that any specific hackers had been identified, although Russia and Iran have previously been accused of trying to meddle in British politics.
Ballots had been expected to arrive from Monday as the two remaining candidates, Liz Truss and Rishi Sunak, make their case to roughly 200,000 Conservative members. Postal ballots could now arrive as late as August 11, members were told.
"We have consulted with the NCSC [National Cyber Security Centre] throughout this process and have decided to enhance security around the ballot process," a party spokesman said. "Eligible members will start receiving ballot packs this week."
A spokesman for the NCSC said it had provided advice on security considerations around online voting.
UK Conservatives on the leadership campaign trail — in pictures
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Candidates Rishi Sunak and Liz Truss at the Conservative Party leadership election hustings at Wembley Arena, London. This was the final hustings attended by Tory Party members who will vote for the new leader and next prime minister. EPA -
Liz Truss addresses Conservative Party members at Wembley Arena. AP -
Mr Sunak puts forward his case to be leader while speaking at Wembley Arena. Bloomberg -
Mr Sunak visits his family's old business, Bassett Pharmacy in Southampton, while on the campaign trail. Reuters -
Ms Truss speaks to staff at Condimentum Ltd at the Food Enterprise Park in Norwich. Getty Images -
A Rishi Sunak supporter at a hustings event in Norfolk. PA -
Mr Sunak greets people at a leadership hustings in Manchester. Bloomberg -
Ms Truss poses for a selfie with a supporter as she arrives for the event in Manchester. Getty -
Rishi Sunak looks through the sights of an anti-tank missile launcher, supplied to Ukraine, during a visit to the Thales Defence System plant in Belfast, Northern Ireland. AP -
Liz Truss at a Conservative Party leadership campaign event in Belfast. Getty -
Liz Truss meets supporters before a hustings in Perth, Scotland. Getty -
Rishi Sunak speaks during a campaign visit to Cluny Castle in Inverurie, Scotland. Getty -
Police hold back protesters outside Perth Concert Hall, Scotland, where Conservative leadership candidates Liz Truss and Rishi Sunak were speaking. PA -
A mural showing the two leadership contestants appears on a wall in Belfast, Northern Ireland. AP -
Rishi Sunak during a visit to St John's Wood Synagogue, north London. PA -
Liz Truss at the Conservative Party leadership election hustings in Cheltenham. EPA -
Rishi Sunak during a hustings event in Cheltenham. Reuters -
Liz Truss speaks to scientists during a campaign visit to a life sciences laboratory at Alderley Park in Manchester. PA -
Rishi Sunak during a campaign hustings in Darlington, England. Bloomberg -
Liz Truss meets party members before she speaks at the Darlington campaign hustings. Getty -
Rishi Sunak arrives for the Darlington hustings. Getty -
Liz Truss plays pool during a visit to the Onside Future Youth Zone in London. Reuters -
Rishi Sunak looks at a book with Teddy Openshaw, 4, as his father, Henry, looks on after a Conservative leadership campaign event in Ribble Valley, Lancashire, England. Getty -
Liz Truss plays table tennis with former Conservative party leader Iain Duncan Smith during a visit to the Onside Future Youth Zone in London. Reuters -
Liz Truss with supporters at a campaign event in Solihull, England. Reuters -
Rishi Sunak attends a campaign event held by his party in Bexhill, England. Reuters -
A protester is removed as Ms Truss speaks during the Conservative Party leadership hustings in Eastbourne. Bloomberg -
Mr Sunak speaks at an event in Edinburgh. PA -
Ms Truss leaves the hall in Exeter, south-west England, after her speech to a Conservative Party membership hustings. Getty -
Mr Sunak speaks at the University of Exeter in south-west England. AP -
A supporter of Ms Truss at a Conservative Party hustings in Exeter, south-west England. Getty -
Ms Truss at a hustings event at the Pavilion conference centre at Elland Road in Leeds. PA -
Mr Sunak in Salisbury watching a screening of the Uefa Women's Euro 2022 final between England and Germany at Wembley stadium. England won 2-1 in extra time. PA -
Ms Truss before the Uefa Women's Euro 2022 final at Wembley stadium in London. AFP -
Mr Sunak meets Tory members at Fontwell Park Racecourse. Reuters -
Ms Truss at an event in Dereham, Norfolk. AFP -
Mr Sunak in Ropley, near Winchester, Hampshire. PA -
Ms Truss speaks as Conservative Member of Parliament Tom Tugendhat looks on at a Conservative Party leadership campaign event at Biggin Hill Airport. Getty -
Ms Truss on the campaign trail in Woodford Green, on the outskirts of London. Reuters -
Mr Sunak at a Conservative Party hustings event in Leeds. AFP -
Ms Truss speaks at the hustings event in Leeds. AFP -
Sunak and Truss-themed souvenirs at a stand during the Leeds hustings event. Reuters -
Ms Truss, and her mother, Patricia, arrive for the hustings event. Getty -
Ms Truss poses with supporters in Morley, England. Getty -
Mr Sunak delivers a speech during a campaign event in Newmarket. AFP -
Ms Truss and Jake Berry, MP for Rossendale and Darwen, during a visit to a broadband interchange company in Leeds. PA -
Mr Sunak answers questions as he takes part in the hustings in Leeds. AFP -
Ms Truss speaks at an event at a private house with members of the Conservative Party in Woodford Green. PA -
Ms Truss and Mr Sunak pose for a picture before a BBC leadership debate at Victoria Hall in Hanley. Getty -
Ms Truss addresses Mr Sunak during the televised debate. Getty -
Mr Sunak, with daughters Krishna and Anushka, and wife Akshata Murthy, during a visit to Vaculug tyre specialists at Gonerby Hill Foot, Grantham. PA -
Ms Truss speaks to people while campaigning in Marden. Getty -
Ms Truss celebrates after being named, along with Mr Sunak, one of the final two candidates in the race to become the UK's next prime minister. PA -
Mr Sunak outside his campaign office in central London. EPA -
Ms Truss outside Downing Street in London. Reuters -
Rishi Sunak meets Conservative Party members and activists, in Teesport, Redcar. Reuters -
Conservative leadership candidates Mr Sunak, Ms Truss, Kemi Badenoch, presenter Julie Etchingham, Tom Tugendhat and Penny Mordaunt during 'Britain's Next Prime Minister: The ITV Debate' in London. EPA -
Ms Badenoch arrives at Here East studios in Stratford, east London, before a live television debate. PA -
Mr Sunak arrives at a London event to launch his campaign to be the next Conservative Party leader and UK prime minister. Reuters -
Mr Tugendhat appears on the BBC One current affairs programme, 'Sunday Morning'. PA -
Ms Badenoch, Ms Mordaunt, Mr Sunak, Ms Truss and Mr Tugendhat before the live television debate. PA -
Ms Mordaunt arrives for the launch of her Conservative leadership campaign at the Cinnamon Club in Westminster. PA -
Ms Truss speaks at the launch event for her campaign to become the next leader of the Tory Party and Britain's prime minister, in London. EPA -
Chancellor of the Exchequer Nadhim Zahawi, one of the candidates who has been knocked out of the race, giving a speech at the Churchill War Rooms. PA -
Former leadership candidate Jeremy Hunt appearing on the BBC One current affairs programme, 'Sunday Morning'. PA -
Sajid Javid attending the launch of his campaign to be Conservative Party leader and prime minister, at the Cinnamon Club in London. He has since been knocked out of the contest. PA -
Rehman Chishti is interviewed after announcing his bid to become the next prime minister. Mr Chishti is no longer in the race. Getty -
Mr Tugendhat speaking at the launch of his campaign. PA -
Culture Secretary Nadine Dorries and Conservative MP Iain Duncan Smith arrive for the Truss campaign launch event. Getty -
Former leadership candidate Attorney General Suella Braverman at the Churchill War Rooms in central London. PA -
Supporters of Mr Sunak wait for his arrival ahead of the campaign launch. Bloomberg -
Mr Tugendhat makes a speech at the launch of his campaign. PA -
Mr Javid mops his brow while launching his campaign. Getty -
Ms Truss leaves for work after her announcement that she would be running for the leadership of the Conservative Party. Getty
The two candidates were in Cardiff on Wednesday for their third official hustings event of the campaign.
At an informal appearance in Shropshire, Ms Truss promised to raise defence spending, raise performance in schools, slim down government agencies and “challenge Treasury orthodoxy” with more investment outside big cities.
Mr Sunak pledged to cut taxes, boost growth and help families hit by the cost of living.
A YouGov poll showed Ms Truss moving into a formidable lead of 69 per cent to 31 per cent, although the race is regarded as difficult to predict because there is little data on the Tory membership.
Asked whether she was looking forward to being prime minister, Ms Truss said: “I am not taking anything for granted. There is still a long way to go in this leadership election.”
Everything you need to know about Liz Truss — video
Ms Truss had endured a difficult day on the campaign trail on Tuesday as an outcry from MPs forced her to abandon a planned pay cut for public sector workers less than 24 hours after announcing it.
It came a day after Mr Sunak was himself accused of a an about-turn by announcing plans for an ambitious tax cut after previously warning against what he called “comforting fairy tales”.
Mr Sunak opened up a new front in the policy debate on Wednesday by announcing his plans on counter-terrorism, saying officials should put more focus on suspected Islamist extremists rather than other groups such as the far right.
He said the definition of extremism would be widened to include “vilification of the United Kingdom”, while offering assurance that this would not be equated with opposing government policy.
Former party treasurer Lord Cruddas, a supporter of outgoing leader Boris Johnson, made an optimistic push to use the delay to balloting to keep the incumbent prime minister in office.
In a letter to the party’s board, he said the contest should be paused and members 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.
