Britain’s election dates are not fixed and the timing of a vote, decided by the prime minister, is subject to fevered speculation, with bookmakers taking bets on possible outcomes.

Craig Williams, an aide of former prime minister Rishi Sunak, bet £100 that Sunak would call an election in July 2024, just days before he did, according to reports.

When asked in June 2024 whether he had placed the bet based on inside information, Williams, 39, said he had “clearly made a huge error of judgement”.

Later that month the Conservative Party withdrew support for Williams, who stood to win £500 from the bet.

He lost his seat in the election.

The gambling commission launched an investigation into people suspected of using confidential knowledge of the election date to gain an unfair advantage in betting markets, an offence under the Gambling Act 2005.

“The commission is committed to upholding the law and will continue to take robust action against illegal betting practices, including investigating and prosecuting offences,” it said in a statement.

The 15 charged will appear at Westminster magistrates’ court on June 13, the gambling commission said.