Senior Conservatives and campaigners are calling for a ban on political bets by MPs, as the Gambling Commission was urged to look into another £500 wager connected with the growing election gambling scandal.

The former Tory leader Iain Duncan Smith said parties should examine the rules when parliament returns amid growing outrage over Tory candidates and aides allegedly staking money on politics. The former defence minister Tobias Ellwood also said there should be new restrictions.

The Conservatives have launched their own inquiry into whether politicians or officials gambled on the timing of the election. Rishi Sunak was forced to deny having placed any bets himself and told reporters he was not aware of any further candidates under scrutiny.

“We’ve been conducting our own internal inquiries and of course will act on any relevant findings or information from that and pass it on to the Gambling Commission,” he said.

The Gambling Commission has been urged to investigate a flurry of unusual activity around the time Sunak called the election, an industry source told the Guardian.

It includes a bet of £504, placed on the Sunday before the election was called, that Sunak would still be PM before the 2024 election. The implication is that the punter may have known that the election call was imminent, thus giving the Tory party no time to replace Sunak.

Marginal odds meant the punter stood to gain only £35 from the bet. Details of the bet have been provided to the commission by a professional gambler.

Earlier this month the Guardian revealed that the gambling watchdog had written to all licensed bookmakers requesting information on anyone who stood to gain more than £199 by betting on the timing of the election. The scope of the investigation is understood to include bets placed against an autumn election as well as those placed on a summer one.

Another gambling industry source said more junior figures may emerge as the commission conducts background checks on the hundreds of people who placed bets on the timing of the poll in the days before Sunak announced the date.

Four Tory candidates and officials are under investigation by the Gambling Commission: Sunak’s top parliamentary aide, Craig Williams, the candidate for Montgomeryshire and Glyndŵr; the Tory candidate in Bristol North West, Laura Saunders; her husband and the party’s director of campaigning, Tony Lee; and the party’s chief data officer, Nick Mason, who has denied wrongdoing.

An unnamed Metropolitan police officer who is part of Sunak’s close protection security team has been arrested in connection with the inquiry.

On Monday evening Steve Baker, the Northern Ireland minister, said he would have suspended anybody who admitted to placing a bet. “I would call them up and ask them ‘did you do it?’ And if they did it then they are suspended,” he told ITV’s Peston.

“But the prime minister would have to answer why he hasn’t done it. I haven’t got inside information on why the prime minister hasn’t done it.”

Duncan Smith said the public took a “dim view” of politicians who gambled on politics. “I am opposed to people associated with politics betting on political outcomes in general because they’re too close to it,” he said. “It also looks casual if you’re betting on stuff that affects people’s quality of life.

“You can either do it by the party saying it won’t be tolerated, or you could do it by legislation. If you start relying on legislation, it would be a complex process. Parties are capable of saying that anyone who does this shouldn’t be in the party. They can move quicker than governments.

“But one way or the other, it needs to be made clear that the public takes a dim view of it and it shouldn’t happen. It’s a matter for the next parliament.”

Ellwood told BBC Radio 4’s Today programme it was comparable to the rules placed on the stock market. “Let’s introduce clear rules, as you have in the City in connection to the purchase of stocks and shares, for example. Let’s prevent any current politician or party professional from placing any bets in the future,” he said.

It is understood that Labour is not planning any action on restricting political betting by its candidates or staff, as long as the bets are not placed with any insider information.

Speaking at a school in Kettering, Keir Starmer said: “I’m not sure we need to start changing the rules. The rules actually aren’t the problem here; there’s a problem with the politicians. The moment the election was called, they didn’t say, ‘Give me a microphone and let me make my case,’; they said, ‘Let me head down to bookies.’”

Other senior politicians and campaigners have joined calls for bets to be restricted. Don Foster, the Liberal Democrat peer and chair of Peers for Gambling Reform, said all betting on elections should be banned.

“The current rules are incredibly opaque and this farrago is a good illustration,” he said. “My personal view is that after the election, we should clarify the rules, including saying that gambling companies should not be allowed to take bets on the outcome of, or anything relating to, elections at any level within the UK.”

Will Prochaska, of the Coalition Against Gambling Ads, said: “Political betting should be banned, along with betting on anything other than sports or horse racing. It’s totally inappropriate for the gambling industry to offer a market for political betting, not least because it tempts those with inside information to try and make a few quid, but also because it normalises the idea that we should bet on anything and everything, which is not normal.

“In the absence of a complete ban, clearly, politicians and their staff should not be able to gamble on political events. We need to protect politics from people who view it as chance to make a quick buck.”