Hundreds of Conservative members have reportedly been sent extra ballot papers, as voting begins in earnest for the new prime minister.
More than a 1,000 people may have received two voting forms for the contest between Boris Johnson and Jeremy Hunt, such as those who have changed their name or joined two Tory associations, according to the BBC.
Former party chairman Sir Patrick McLoughlin, who is running Mr Hunt’s bid, said the issue has “got to be looked at” but Tory members should follow the rules and vote just once.
The party said there were “clear instructions” that anyone found to have voted more than once would be expelled.
It comes as a new poll put Mr Johnson on course for a landslide victory, with the backing of 74 per cent of Conservative members compared to 26 per cent who prefer Mr Hunt.
Ballot papers have been sent to around 160,000 members ahead of the voting deadline on 22 July. The winner will be announced the following day.
Sir Patrick, a former cabinet minister, told the Today programme: “It’s made very clear on the ballot paper that you are only allowed to vote once. I expect Conservative members to follow that.
“If people have joined two associations they may get two ballot papers.
“I get two ballot papers in elections because I am on two different sets of electoral registers, it doesn’t mean that I vote twice, I don’t.”
He added: “It can’t be clearer, it’s right there on the ballot paper saying that you must only vote on one occasion and I expect people to do that.”
Tory grandee Iain Duncan Smith, chairman of Mr Johnson’s campaign, said the party needed to ensure it could sift the ballots properly.
“Obviously there is an issue here, I accept that,” he said, adding that he was sure party chairman Brandon Lewis was looking at it.
Mr Duncan Smith said: “In all elections there are issues about this, where people are registered separately, and in the ballot papers it’s quite clear ... that you are not to vote more than once.
“But it’s clear that the Conservative Party needs to continue to do some work and look into how they can actually sift this properly so that they recognise who is who.”
Mr Hunt faces an uphill battle to knock his rival off course, as a YouGov survey for The Times gave him only a quarter of the support from Tory members.
The foreign secretary urged activists to “try before you buy” by waiting for the TV debates next week, as he scrambles to make up ground.
But allies of Mr Johnson have been urging members to vote immediately, using a social media campaign with the slogan, “To get Boris, vote Boris”.
Both men were due to appear at a hustings in Nottingham and Cardiff on Saturday.
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