The Gambling Commission has spoken to the Prime Minister’s chief of staff as a witness in connection with bets allegedly placed on the date of the General Election, it has been reported.

The BBC said Liam Booth-Smith was interviewed by the regulator last week to clarify who may have known about the date of the General Election before it was announced.

The Gambling Commission investigation is focused on allegations of cheating, while Scotland Yard will lead on what is likely to be a much smaller number of cases where there could be additional offences such as misconduct in public office.

An update by the Metropolitan Police and Gambling Commission said at least seven officers from the force are being investigated.

Five Conservatives have also been caught up in the inquiry, with reports suggesting the figure could be 15 parliamentary candidates and officials, although the gambling watchdog has not confirmed the numbers involved.

Commission chief executive Andrew Rhodes said: “We are focused on an investigation into confidential information being used to gain an unfair advantage when betting on the date of the General Election.

“Our enforcement team has made rapid progress so far and will continue to work closely with the Metropolitan Police to draw this case to a just conclusion.”

Rishi Sunak has withdrawn Tory support for his former parliamentary aide Craig Williams’s bid to be returned as MP for the Montgomeryshire and Glyndwr seat, after he admitted having a “flutter” on the election date.

During a campaign visit in Derbyshire, Mr Sunak was repeatedly asked whether he had confided in Mr Williams ahead of his surprise announcement of a summer election.

Speaking to broadcasters, the Prime Minister said: “I’ve been clear about this. I’m furious to have learnt about these allegations.

“We’ve initiated independent inquiries of our own, because I don’t have access to the Gambling Commission’s detail.

“You’ll recognise that while there are ongoing independent investigations, it’s just not right for me to say anything more about that.”

The Tories have also withdrawn support from Bristol North West candidate Laura Saunders whose husband, Tony Lee, the Conservative Party’s director of campaigning, has taken a leave of absence, as has Tory chief data officer Nick Mason.