Rishi Sunak has become mired in a row over alleged betting on the general election date after it emerged that a second Conservative candidate and the party’s campaigns director were being looked into by the Gambling Commission.

The watchdog is examining bets allegedly placed by Laura Saunders, the Tory candidate in Bristol North West, and her husband, Tony Lee, who is now on leave of absence from his job at party headquarters.

It is not known when the alleged bets were placed or for how much money.

Labour called for the Tories to suspend Saunders from the party. The Labour leader, Keir Starmer, said: “This candidate should be suspended and it’s very telling that Rishi Sunak has not already done that. If it was one of my candidates, they’d be gone and their feet would not have touched the floor.”

The Liberal Democrats called for a Cabinet Office inquiry. The party’s leader, Ed Davey, said: “I think it’s quite awful. It needs a heavy hand from the top.”

The cabinet minister Michael Gove said the situation “doesn’t look great” for the Conservatives but said the commission had to get on with its investigation.

A Tory spokesperson said the party had been contacted by the watchdog about “a small number of individuals” who had allegedly placed bets on a July election. A party insider said they were not aware of any more cases beyond those already in the public domain.

Last week the Guardian revealed that the prime minister’s closest parliamentary aide, Craig Williams, placed a £100 bet with Ladbrokes at 5/1 on a July poll, three days before Sunak announced the date.

The Guardian later revealed that the Gambling Commission asked bookmakers to trawl through all substantial bets placed on a July election.

The watchdog wrote to all licensed bookmakers requesting information on anyone who stood to gain more than £199 by betting on a July election in the UK.

Saunders, who has worked for the Tories since 2015, is standing as the party’s candidate in Bristol North West, which Labour held on the old boundaries with a 5,700 majority. She has not yet commented on the allegations.

Williams, 39, is the Tory candidate for Montgomeryshire and Glyndŵr in mid-Wales. He had a majority of 12,000 before the boundary change.

After the Guardian’s revelation, he apologised for placing the bet three days before Sunak announced the date, saying he had made a “huge error of judgment”. The prime minister said he was “disappointed”.

The Guardian understands the Tory party opened a formal HR process when they were notified of the watchdog’s inquiries, but has now put that on hold. Lee is on a leave of absence from party headquarters.

The Tories on Thursday deleted a campaign video that showed a roulette table spinning with the words: “If you bet on Labour, you can never win.”

Separately, one of Sunak’s close protection officers was arrested on Monday over alleged bets about the timing of the election, on suspicion of misconduct in public office.

The Metropolitan police said they were contacted last Friday by the commission, which informed the force that it was looking into alleged bets related to the timing of an election by a constable from the Met’s royalty and specialist protection command.

“The matter was immediately referred to officers in the Met’s directorate of professional standards, who opened an investigation, and the officer was also removed from operational duties,” the force said.

The matter has also been referred to the Independent Office for Police Conduct (IOPC), the watchdog responsible for overseeing the system for handling complaints made against police forces in England and Wales.

The Met added: “The Gambling Commission continues to lead the investigation into the alleged betting offences, and our investigation is running in parallel to that.”

The commission said: “Currently the commission is investigating the possibility of offences concerning the date of the election.

“This is an ongoing investigation, and the commission cannot provide any further details at this time. We are not confirming or denying the identity of any individuals involved in this investigation.”

Labour’s campaign chief, Pat McFadden, who has written to Sunak urging the Tories to withdraw support from Saunders and Williams while they are being looked into, said the Tories needed to say “how wide this scandal goes”.

He added: “This looks as though there is one rule for members of the Tory party, and another rule for everyone else, specifically on this occasion a serving police officer.”