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.

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.

The Gambling Commission and the Met are focusing on whether the alleged use of confidential information constitutes cheating under criminal law.

Six Met officers had already been caught up in the scandal, with one, a protection officer for the prime minister, 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 – would be investigated by the Gambling Commission.

The Guardian uncovered the gambling scandal two weeks ago, revealing that Craig Williams, Rishi Sunak’s closest aide, was the subject of an investigation by the Gambling Commission for betting that the election would be in July, three days before it was called.

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.

The decision to drop Williams and Saunders as candidates on Tuesday was Sunak’s latest effort to draw a line under the gambling scandal that has dogged his election campaign.

The Guardian also revealed that Russell George, a Tory member of the Welsh parliament who represents the same constituency as Craig Williams, is part of the commission’s inquiry.

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.”