Rishi Sunak said Tory figures who are found to have broken gambling rules “should face the full force of the law”, as he is likely to face further questions on the betting scandal engulfing his faltering election campaign.

The Prime Minister said he was “incredibly angry to learn” of the allegations that a string of people with links to the Conservative Party or No 10 bet on the timing of the July 4 contest before he announced it.

Concerns over political insiders profiting from the election date could overshadow his campaigning on Friday, when he is at the Welsh Conservative manifesto launch in Kinmel Bay.

Mr Sunak was asked during a bruising appearance on Thursday night’s BBC Question Time leaders’ special whether the betting allegations were “the absolute epitome of the lack of ethics” displayed by the Conservative Party in recent years.

The Prime Minister replied: “I was incredibly angry – incredibly angry – to learn of these allegations.

“It’s a really serious matter. It’s right that they’re being investigated properly by the relevant law enforcement authorities, including … a criminal investigation by the police.

“I want to be crystal clear that if anyone has broken the rules, they should face the full force of the law.”

Conservative candidate Laura Saunders said she “will be co-operating with the Gambling Commission” probe, while her husband, the Tories’ director of campaigning, Tony Lee, took a leave of absence amid reports the couple were being investigated by the gambling regulator.

It came after the arrest of one of the Prime Minister’s police protection officers and the previous revelation of a Gambling Commission investigation into his parliamentary aide, Craig Williams.

Labour and the Lib Dems have called on the Tories to suspend both Ms Saunders and Mr Williams, who is standing in Montgomeryshire and Glyndwr.

Pressed on those demands, Mr Sunak said the investigations were ongoing and the “integrity of that process must be respected”.

But, he added: “What I can tell you is if anyone is found to have broken the rules, not only should they face the full consequences of the law, I will make sure that they are booted out of the Conservative Party.”

Betfair data appears to show a flurry of bets on a July poll placed on May 21, the day before Mr Sunak called the election, including some in the hundreds of pounds at odds that would have delivered profits in the thousands.

Despite the Conservatives’ dismal poll ratings failing to improve since Mr Sunak made the move – with four major polls this week even predicting the worst Tory electoral result ever – the Prime Minister insisted he had chosen the “right moment” for the contest.

On Friday, he will ramp up his warnings about handing Labour “a blank cheque” at the election, claiming Sir Keir Starmer’s party will “change every rule they can” to ensure they stay in government once they gain power.

Meanwhile, Sir Keir said Jeremy Corbyn would have been a better prime minister than Boris Johnson.

The Labour leader is heading north of the border on Friday to pledge that Labour’s industrial strategy would deliver 69,000 jobs in Scotland as he seeks to cement his party’s resurgence there.

Visiting a business alongside Scottish Labour leader Anas Sarwar, Sir Keir will vow to “turn the page on an era of economic turmoil under the Tories and the SNP”.

In his Question Time grilling, the Labour leader ducked a volley of questions over whether he truly believed his predecessor would make a “great” premier.

When host Fiona Bruce asked for a “yes or no” answer to whether he meant his one-time statement, there was laughter from the audience when he did not give one, instead saying that Mr Corbyn would have made a better prime minister than Mr Johnson.

Sir Ed was challenged by a student over the Lib Dems abandoning their pledge to scrap tuition fees in the coalition government.

He said: “I understand why your generation lost faith in us. It was a difficult government to be in.”

The Lib Dem leader was also asked whether he was “proud” of his conduct as postal affairs minister between 2010 and 2012.

He said he made “two big mistakes during that time”, including failing to meet Sir Alan Bates in 2010 – although he subsequently did agree to meet the campaigner – and not seeing through assurances given to him by the Post Office that Sir Alan’s assertions were not true.

The Lib Dem leader will call for urgent reform to end the “appalling situation” in NHS dentistry as he hits the campaign trail on Friday.

Scotland’s First Minister and SNP leader, John Swinney, suggested he would prefer Sir Keir over Mr Sunak, telling the BBC programme: “The Conservative Government has been a total disaster and a calamity”.

Reform UK leader Nigel Farage, who complained about being excluded from the four-way leaders’ panel but will feature in a separate Question Time next week, is expected to campaign in Clacton on Friday before a high-profile Panorama interview.