Rishi Sunak’s closest parliamentary aide placed a £100 bet on a July election just three days before the prime minister named the date, the Guardian can reveal. The Gambling Commission is understood to have launched an inquiry after Craig Williams, the prime minister’s parliamentary private secretary, who became an MP in 2019, placed a bet with the bookmaker Ladbrokes on Sunday 19 May in his local constituency of Montgomeryshire and Glyndŵr. On 22 May, Sunak made the surprise announcement that a general election would be held on 4 July. In a statement, Williams said: “I’ve been contacted by a journalist about Gambling Commission inquiries into one of my accounts and thought it best to be totally transparent. “I put a flutter on the general election some weeks ago. This has resulted in some routine inquiries and I confirm I will fully cooperate with these. “I don’t want it to be a distraction from the campaign, I should have thought how it looks.” A Conservative party spokesperson added: “We are aware of contact between a Conservative candidate and the Gambling Commission. “It is a personal matter for the individual in question. As the Gambling Commission is an independent body, it wouldn’t be proper to comment further, until any process is concluded.” It is understood that a red flag was automatically raised by Ladbrokes as the bet in Williams’ name was potentially placed by a “politically exposed person”, and the bookmaker is particularly cautious over “novelty” markets. The £100 bet, which could have led to a £500 payout on odds of 5/1, is believed to have been placed via an online account that would have required the user to provide personal details including their date of birth and debit card. The bookmaker also knows the location of the bet. Ladbrokes referred the case to the Gambling Commission, which is understood to have launched an inquiry. The Guardian understands the regulator informed Downing Street officials last week. Using confidential information to gain an unfair advantage when betting may constitute a criminal offence. Separately, the MPs’ code of conduct prohibits members from “causing significant damage to the reputation and integrity of the house”. The bet was placed while parliament was still in session. Jonathan Ashworth, Labour’s shadow paymaster general, said: “These allegations are utterly extraordinary. Rishi Sunak has sat on this information for more than a week but has lacked any backbone to take action. Once again Rishi Sunak has been exposed as utterly weak. “After all the Tory financial scandals, this is more evidence that the Tories have learned nothing, haven’t changed, and if given five more years, the chaos will just continue.” While Sunak’s general election announcement last month came as a surprise to the public and many Conservative MPs, the prime minister is thought to have been considering and debating the timing for months. He is believed to have settled on the July date weeks before confirming it in heavy rain outside Downing Street. The alleged bet, which would not have received a payout until after the election took place, is likely to be highly embarrassing for Sunak, who has been accused of presiding over a calamitous general election campaign. Sunak and Williams will now face questions about who knew about the election date and when. Williams, 39, is the Tory candidate for Montgomeryshire and Glyndwr in mid-Wales. He had a majority of 12,000 before the boundary change. His role as parliamentary private secretary is to be the prime minister’s eyes and ears in parliament. He is understood to have been in Downing Street on an almost daily basis, and to be a trusted member of Sunak’s team. Ladbrokes declined to comment. A spokesperson for the Gambling Commission said: “If someone uses confidential information in order to gain an unfair advantage when betting, this may constitute an offence of cheating under section 42 of the Gambling Act, which is a criminal offence. “The Gambling Commission does not typically confirm or deny whether any investigations are under way unless or until they are concluded, or if arrests are made or charges are brought during a criminal investigation.” Many Tory MPs were blindsided by Sunak’s decision to call a summer election. After he ruled one out in May, most had assumed that his plan to hold the vote in the “second half of the year” meant that it would be in the autumn. The prime minister is said to have opted for a pre-summer election in April, when it became clear that growth figures were going to show inflation falling and an economy returning to better health. He was said to be concerned that the public would grow increasingly frustrated if he did not name a date. Oliver Dowden, the deputy prime minister; Liam Booth-Smith, Sunak’s chief of staff; and James Forsyth, his political secretary, were at the heart of the deliberations. Isaac Levido, who is running the summer campaign, was one of those arguing against an early election, according to sources. They said David Cameron, the foreign secretary, and Jeremy Hunt, the chancellor, were told only the night before Sunak’s announcement, while the rest of the cabinet were informed just before it. A Downing Street spokesperson declined to comment.