Rishi Sunak backed the “full force of the law” being thrown against any Tory figures if they were found to have illegally bet on the election day as the row threatened to inflict another blow on his struggling campaign.
The Prime Minister told how he was “incredibly angry” to learn of the startling allegations swirling over No10 and Tory HQ.
However, he stopped short of suspending the two parliamentary candidates facing the claims.
He was grilled on the furore when he appeared on BBC Question Time for 30 minutes of questions.
Sir Keir Starmer, Scottish National Party leader John Swinney and Liberal Democrat leader Sir Ed Davey also fielded questions for half an hour.
But Mr Sunak came under the most pressure and at times showed signs of frustration as audience members quizzed him.
The PM said anyone found to have broken the rules will be “booted out of the Conservative Party” amid the election betting scandal.
During the BBC’s Question Time special, the Prime Minister stressed: “These investigations are ongoing, they are widely confidential, one of them is a criminal investigation that’s being conducted by the police.”
Mr Sunak said the “integrity of that process should be respected”, adding: “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.”
However, unlike the Met Police, which has acted against an officer facing a claim in the betting row, the Tories have not suspended two parliamentary candidates, Craig Williams and Laura Saunders, who are facing similar allegations.
Before Mr Sunak’s grilling, Sir Keir gave a pledge to get the NHS waiting list backlog cleared within five years.
But he struggled once again to give a convincing answer over why he said Jeremy Corbyn would make a “great Prime Minister” when the Leftwinger was party leader and Sir Keir served in the shadow cabinet.
He was challenged over why his party would not put a goal on cutting levels of immigration.
He also told how he was “genuinely worried” about the rise of Right-wing leaders and movements in other European countries, and stressed when dealing with them he would make the argument as a “progressive”.
He was also grilled over why Labour had backtracked on its plans to reduce university tuition fees.
“Every single politician who has put a number on it has never met that number,” he told the BBC Question Time audience.
The Labour leader added: “We want to get it down significantly. It needs to be balanced immigration so it works for our economy and works for our country.
“We need to get it down, but if we are going to do that, we need to understand what the problem is.”
Sir Keir pointed to training more people in various skills as one means of reducing net migration.
Earlier, Sir Ed Davey faced tough questions about his record in the coalition years and as minister in the Horizon scandal.
The Liberal Democrat leader went first in the BBC Question Time special.
Challenged by a student over the Lib Dems abandoning their pledge to scrap tuition fees in the coalition government, Sir Ed said: “I understand why your generation lost faith in us. It was a difficult government to be in.”
He said the loss of trust in his party after that period was “very scarring” and admitted he was “not proud” of some of the votes he had to take part in.
Sir Ed was also asked whether he was “proud” of his conduct as postal affairs minister under the coalition government between 2010 and 2012.
He has recently come under fire for failing to do more to help wrongly-convicted subpostmasters when he held the brief and for initially refusing to meet Alan Bates in 2010.
Sir Ed said he made “two big mistakes during that time”, including failing to meet the campaigner and not seeing through assurances given to him by the Post Office that Mr Bates’s assertions were not true.
“I’m sorry for not seeing through those lies. There were many ministers of all political parties during this 20-year period who didn’t see through those lies.”
He said he hoped “people go to prison” over “the biggest miscarriage of justice in our country’s history”.
Mr Swinney, asked whether he was going to carry on with independence referenda “until you get the answer you want”, stressed his belief was that Scotland would be better as an independent country.
Pressed on how he would boost the manufacturing sector, Mr Swinney said Brexit was the “source of the problems”, and that rejoining the EU would provide a boost to the country’s manufacturing sector.
There was no spot on the programme for Reform UK’s Nigel Farage, who previously demanded a place given his party’s strong showing in the opinion polls.
