Rishi Sunak’s closest parliamentary aide, Craig Williams, has apologised for placing a £100 bet on a July election three days before the prime minister announced the date for the poll. Speaking to the BBC, Williams, who is standing for re-election in Montgomeryshire and Glyndŵr, said: “I’ve clearly made a huge error of judgment, that’s for sure. And I apologise.” The apology goes further than his statement on Wednesday when the Guardian revealed he had placed a £100 bet with Ladbrokes at 5-1. The bet is now the subject of an investigation by the Gambling Commission. Williams, who is Sunak’s parliamentary private secretary, admitted on Wednesday to a “flutter” on one of his betting accounts and conceded: “I should have thought how it looks.” But Williams has refused to say whether he placed the bet on the basis of insider information about when Sunak was planning to hold the election. In response to questions about this, he told the BBC: “As I have said, I will not be expanding on that statement. I’m not expanding because it’s an independent process. The Gambling Commission are looking at it now. And all I can say was it was a huge error of judgment. A huge error of judgment and I won’t be adding to the statement I’ve already made.” David Cameron, the foreign secretary, condemned Williams’ bet as “very foolish”. Asked if he was happy for Williams to stand as a Conservative candidate, Cameron said “yes” but qualified this by saying it was too late for the party to remove Williams from ballot papers. “He’s going to be investigated, and will have to face the consequences of that investigation,” Cameron said. Prof Jon Tonge, who teaches British politics at the University of Liverpool and regularly places political bets, urged Williams to make clear if he had made any other bets on the date of the poll. He said: “One of the questions it raises is did he try to bet more with other accounts, because a £100 bet is probably the maximum that you could put on a bet like that, with a single bookmaker, before you start raising suspicions. “I’m absolutely certain the Gambling Commission will want him to come clean about whether he placed any other bets on the same outcome.” Tonge added: “He [Williams] may claim that it was a hunch it wasn’t the product of any inside information. And it’s true that Rishi Sunak kept the election date very secret, and most Conservative MPs were taken completely off guard. “If he knew the date he should probably have been alerting his fellow Conservative members, rather than making a quick bet with the bookies. “I think it was very, very foolish because there was every chance of a bet like that leaking out. The optics of it are awful. He should have been more self disciplined and not put it on. It is yet another act of political folly from the Conservatives during a wretched campaign.” Tonge said the bet was “potentially the political version of insider trading” if it could be proved Williams knew of the date of the election. But he added: “It would be very difficult to prove unless there’s a communication trail that shows that he was in possession of the knowledge of the election date.”