The Metropolitan police have indicated that the dropped Conservative candidate Craig Williams could come under the scope of a criminal investigation into betting on the election that has overshadowed Rishi Sunak’s campaign.

Scotland Yard will investigate any suspicious bets that could represent a misconduct in public office offence, while the Gambling Commission will continue to look at whether betting rules were broken.

The prime minister repeatedly refused to say whether he told Williams, his closest parliamentary aide, about the date of the election, wrongly claiming he could prejudice the watchdog’s inquiry.

So far, five Conservatives are known to have been caught up in the Gambling Commission inquiry since the Guardian revealed two weeks ago that Williams had placed a £100 bet with Ladbrokes three days before Sunak announced the date.

The Tories have withdrawn the party’s support for Williams’ campaign to be returned as the MP for the Montgomeryshire and Glyndwr seat, after he admitted having a “flutter” on the election date.

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

“I’ve been clear about this. I’m furious to have learned about these allegations,” he said. “We’ve initiated independent inquiries of our own, because I don’t have access to the Gambling Commission’s detail.”

The prime minister was told he could not prejudice the investigation and that he was free to answer the question. However, he responded: “You’ll recognise that while there are ongoing independent investigations, it’s just not right for me to say anything more about that.”

The number of Metropolitan police officers under investigation over bets on the timing of the general election has risen to at least seven, the force has said.

Britain’s biggest police force said it would continue to investigate a “small number” of wagers as part of a joint investigation led by the Gambling Commission, which said it was making “rapid progress”.

Sources familiar with the investigation suggested the decision to run parallel inquires could be an attempt to dampen the frenzy around MPs betting on themselves to win or lose their seats, which may be considered unwise but is not illegal.

One Met officer, a protection officer for the prime minister, is under investigation for misconduct in public office after allegedly placing bets on the election date.

The close protection officer has been bailed and is subject to restricted duties, the Met said. It confirmed that “at least” seven officers were under investigation by the Met’s specialist crime command.

In a statement, the Met said the majority of cases – believed to be those without specific features that could mean additional offences apply, such as misconduct in public office – would be investigated by the Gambling Commission.

The watchdog is also examining bets allegedly placed by Tony Lee, the Conservative party’s campaigns director; his wife, Laura Saunders, the Tory candidate in Bristol North West; and Nick Mason, the Tories’ chief data officer.

As many as 15 Conservative candidates and officials are believed to be under investigation over suspicious bets on the date of the election.

Andrew Rhodes, the chief executive of the Gambling Commission, 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.”

Det Supt Katherine Goodwin, who is leading the Met investigation, 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. We will aim to provide updates at key points as our investigation progresses.”