Rishi Sunak has said he is “not aware” of any other Tory candidates facing investigation by the gambling watchdog over allegations of betting on the General Election.

The Prime Minister said the Conservatives had been carrying out their own internal probe running alongside the inquiry and “will act” if it finds any wrongdoing over the scandal that has engulfed the party.

With the Tories still behind in the polls and after a bruising few days dominated by the controversy, Mr Sunak is facing pressure including from the party ranks to take a more robust stance against those involved.

“(The Gambling Commission) don’t talk about the individuals that they are investigating,” the Prime Minister told journalists on the campaign trail in Edinburgh on Monday.

“What I can tell you is I am not aware of any other candidate that they are looking at.”

He added: “What I can tell you is, in parallel, 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.”

Labour leader Sir Keir Starmer said the revelation that the Tories are conducting their own parallel investigation was designed to delay further fallout over the controversy.

Mr Sunak ruled out himself and family members of being the subject of investigation and said he had never placed a bet on politics while serving as an MP.

The faltering Conservative campaign has been plunged into a deeper crisis by the gambling controversy, over which four Tory officials and candidates are being investigated.

The party’s chief data officer Nick Mason is the latest figure understood to be taking a leave of absence amid claims he placed bets on the election date, along with director of campaigning Tony Lee.

Mr Lee’s would-be MP wife Laura Saunders and fellow candidate Craig Williams, who was a parliamentary aide to the Prime Minister, are also under investigation by the Gambling Commission.

Facing questions from broadcasters on Monday morning, Northern Ireland Secretary Chris Heaton-Harris rejected calls, including from within Tory ranks, for those facing a probe to have party backing withdrawn while it is ongoing.

“I think what you’re trying to suggest is that someone is guilty until they’re proven innocent and that is not how this works,” he told BBC Radio 4’s Today programme, when asked why tougher action had not been taken.

Former defence minister Tobias Ellwood on Monday joined a growing chorus of senior Tory voices calling for the Prime Minister to take action.

The Conservative candidate for Bournemouth East said the scandal was a “deeply unhelpful, self-inflicted distraction” and suggested the Prime Minister should have suspended those being investigated.

Sir Keir said Mr Sunak’s announcement of an internal probe was a bid to kick the story “into the long grass” while Liberal Democrat leader Sir Ed Davey said it was “one rule for the Conservatives and another for others”.

Speaking to reporters on the campaign trail in Northampton, the Labour leader said an investigation should have happened already, adding: “It would take half an hour. Who knew? Did you place a bet? That’s it.”

Mr Sunak sought to move on from the fallout with a rallying cry to Tory activists at the party’s Scottish manifesto launch in Edinburgh on Monday as polling day approaches.

He said it would take decades to recover from the “disaster” of a Labour government as he urged members to fight for every vote in the closing stages of the election campaign.

The Prime Minister said only the Scottish Conservatives have the “courage to stand up to the nationalists” north of the border, as he attacked both the SNP and Labour in his speech.

Rishi Sunak walks alongside seated people with an election poster in the background
Prime Minister Rishi Sunak at the launch of the Scottish Conservative manifesto in Edinburgh (Jane Barlow/PA)

“If the SNP win the majority of seats at this election, they will treat that as a mandate to carry on campaigning for independence for another five years,” he said.

“A vote for the Scottish Conservatives is a vote to put this issue to bed, to move past these tired and stale arguments and to go forward united and together.”

The Prime Minister accused Labour of a potential “sellout” to the European Union, saying: “It is only the Scottish Conservatives that will stand up for the North Sea’s fishing industry, too.

“The EU are already making clear that the price of any deal that Labour wants is a sellout of our fishing industry, and who doubts that Keir Starmer would pay that?”

Labour has said it has no plans to rejoin the EU amid amid Tory suggestions that Brexit would be at risk under a government led by Sir Keir.

The SNP accused the Conservatives of having an “obsession” with it, with the party’s candidate for Gordon and Buchan, Richard Thomson, saying: “The Tory manifesto mentioned the SNP 88 times in 76 pages.

“They are rattled because they know a well deserved democratic drubbing is on its way and, here in Scotland, a vote for the SNP is the best way to get rid of every single Tory MP.”

Mr Sunak and the Labour leader will face questions from Sun readers on Monday in the latest set piece of the election campaign.

Labour will focus on health issues, raising concerns about “DIY dentistry” because of a lack of NHS treatment.

Shadow health secretary Wes Streeting said: “It’s virtually impossible to get a dentist appointment after 14 years of Conservative neglect.

“People are resorting to pulling their own teeth out – DIY dentistry should be the stuff of Charles Dickens’ books, not Britain in 2024.”