The prime minister has refused to say whether he told his parliamentary aide, Craig Williams, about the date of the general election.

The Conservatives withdrew support for Mr Williams as a candidate on Tuesday, after he admitted placing a bet on the election date. He has denied committing any offence.

Earlier, the Metropolitan Police disclosed that the number of its officers under investigation over bets on the timing of the election had risen from six to at least seven.

A royalty and specialist protection officer arrested on suspicion of misconduct in public office remains on bail and is subject to restricted duties while a further six officers have been identified as having placed bets on the timing, the Met said.

The head of the Gambling Commission, which is conducting an investigation alongside the Met, said it would not confirm the names of those under suspicion to “protect the integrity of the investigation”.

In the face of repeated questions about Mr Williams during a campaign visit in Derbyshire, Mr Sunak insisted it would not be right for him to say more about the ongoing investigations.

He said he was furious when he learned about the allegations and had made it clear that anyone who had broken the rules should face the full force of the law.

On Tuesday, the party withdrew its support for Mr Williams and another election candidate, Laura Saunders.

Mr Sunak told reporters he was not aware of any election candidates or Conservative officials being investigated beyond those already in the public domain.

Labour has suspended one of its candidates, Kevin Craig, who is being looked into by the commission for betting against himself in the election.

Separately, it has been reported that a senior Conservative placed an £8,000 bet that he would lose his seat.

Sir Philip Davies has neither confirmed nor denied the bet, which was first reported by the Sun, but denied doing “anything illegal”.

The latest statements from the force and the commission suggest that where there is suspicion of cheating alone, the betting industry watchdog will investigate.

However, where there is a suspicion of an additional offence – such as misconduct in public office – then the Met’s specialist crime command will be brought in too.

It is understood that the number of cases the Met will take the lead on is “very small”.

The definition of who is a public officer includes police constables, elected officials such as MPs, councillors, ministers and mayors, and civil servants.

Detective Superintendent Katherine Goodwin said: “We have agreed a joint approach with the Gambling Commission, who are the appropriate authority to investigate the majority of these allegations.

“There will, however, be a small number of cases where a broader criminal investigation by the police is required.”

Gambling 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.

“We understand the desire for information; however, to protect the integrity of the investigation and to ensure a fair and just outcome, we are unable to comment further at this time, including the name of any person who may be under suspicion.”

A Conservative politician in the Welsh Parliament, Russell George, is also facing inquiries by the regulator and has “stepped back” from his frontbench role.

BBC Newsnight understands that as many as 15 Conservative Party candidates and officials are being scrutinised by the commission.

You can find a full list of candidates for the Bristol North West, Central Suffolk and North Ipswich, and Montgomeryshire and Glyndŵr constituencies on the BBC News website.