Rishi Sunak’s chief of staff has been interviewed as a witness by the Gambling Commission, about the saga over alleged bets on the timing of the general election.

Liam Booth-Smith spoke to the betting regulator at its request last week.

Sources stress that Mr Booth-Smith, the prime minister’s closest aide, is not a suspect in the investigation and had not placed a bet himself.

He was asked to help the commission understand who may and may not have known about when polling day would be.

The ongoing saga started more than two weeks ago, when Tory candidate Craig Williams, who was an aide to the prime minister in the last Parliament, was alleged to have bet on the election date.

Mr Williams has apologised and said he made a “huge error of judgement” by placing a £100 bet on a July election three days before Rishi Sunak announced the 4 July poll.

Since then, more candidates have been revealed as being under investigation, as well as seven Met Police officers.

BBC Newsnight understands that as many as 15 Conservative Party candidates and officials are now being scrutinised by the commission.

Labour has suspended its candidate in Central Suffolk and North Ipswich, Kevin Craig.

On Tuesday, after days of questions, the Tories withdrew support for two election candidates – Laura Saunders in Bristol North West and Craig Williams in Montgomeryshire and Glyndwr.

The BBC’s head-to-head TV debate on Wednesday kicked off with the two leaders being questioned about the claims.

Sir Keir Starmer accused Prime Minister Rishi Sunak of having to be “bullied” into taking action. Mr Sunak said he had handled the affair properly.