Rishi Sunak’s most senior adviser has been interviewed as a witness by officials at the Gambling Commission as part of its investigation into widespread betting by Westminster figures on the date and outcome of the general election. Liam Booth-Smith, the prime minister’s chief of staff, spoke to the regulator last week after a series of revelations about betting by people close to the prime minister on the date of the election. Sources told the BBC that Booth-Smith was not a suspect and had not placed a bet himself. The person said: “We’ve always been clear that we would assist the Gambling Commission with its investigation.” The scandal began two weeks ago, when the Guardian revealed that Craig Williams, Sunak’s parliamentary aide, was under investigation by the commission for placing a £100 bet that the election would be in July, three days before it was announced. The watchdog is also examining bets allegedly placed by Tony Lee, the Conservative party’s campaigns director; his wife, Laura Saunders, the Tory candidate in Bristol North West; and Nick Mason, the Tories’ chief data officer. As many as 15 Conservative candidates and officials are believed to be under investigation over suspicious bets on the date of the election. The Tories have now suspended their support for both Williams and Saunders as candidates. A Labour candidate, Kevin Craig, has also lost the support of his party after it emerged he is also under investigation for placing a bet against himself in his own constituency, betting that the Conservative candidate would win instead. While the gambling regulator conducts its investigation, the Metropolitan police have also indicated that Williams could come under criminal investigation. Scotland Yard said on Thursday it would investigate any suspicious bets that could represent a misconduct in public office offence, in an inquiry that will run in parallel to the Gambling Commission’s own investigation into whether betting rules have been broken. Det Supt Katherine Goodwin, who is leading the Met police investigation, said: “We have agreed a joint approach with the Gambling Commission, who are the appropriate authority to investigate the majority of these allegations. There will, however, be a small number of cases where a broader criminal investigation by the police is required. We will aim to provide updates at key points as our investigation progresses.” Sunak has come under pressure in recent days to say whether he told any Conservative candidates or police officers the date of the election before he announced it publicly. During a campaign visit in Derbyshire on Thursday, Sunak was asked repeatedly whether he had told Williams before making his surprise announcement. “I’ve been clear about this. I’m furious to have learned about these allegations,” he said. “We’ve initiated independent inquiries of our own, because I don’t have access to the Gambling Commission’s detail.” The prime minister was told he could not prejudice the investigation, but would only add: “You’ll recognise that while there are ongoing independent investigations, it’s just not right for me to say anything more about that.”