Rishi Sunak has insisted it is “proper” to wait for the outcome of multiple investigations into allegations of betting on the General Election before taking action.

The Prime Minister faced yet more questions on the scandal that has engulfed his party over the past week as he underwent a grilling on The Sun newspaper’s Never Mind The Ballots show.

But he stuck to his line that he would “not hesitate to act” if either external investigations or the Conservatives’ own internal inquiry found any evidence of wrongdoing.

He said: “What’s proper is to investigate things, to get to the bottom of it, to get to the truth and then act on those findings.”

The Gambling Commission, the police and the Conservative Party are all understood to be looking into claims that several figures with links to the Tories or Number 10 placed bets on the date of the General Election shortly before Mr Sunak announced his plans last month.

Those include Mr Sunak’s former parliamentary aide Craig Williams, who has already admitted to having “a flutter” on the election date in what he told the BBC was a “huge error of judgment”.

The party’s chief data officer, Nick Mason, and campaigning director, Tony Lee, have both taken leaves of absence after being implicated in the scandal, while Mr Lee’s wife, Tory candidate Laura Saunders, is also under investigation by the Gambling Commission.

Earlier on Monday, Mr Sunak said he was “not aware” of any other Tories facing investigation over similar allegations.

After his appearance at The Sun’s event, Mr Sunak made a campaign stop in Chelsea, where he told activists in the usually safe Conservative seat that he understood why people were hesitating to support the Tories.

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 implicated in the gambling scandal.

But Northern Ireland Secretary Chris Heaton-Harris rejected calls for those facing a probe to have party backing withdrawn while it is ongoing.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.

Labour leader Sir Keir Starmer 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, Sir Keir 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.

“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 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.”