One of Rishi Sunak’s closest parliamentary aides has said he made a “huge error of judgment” by betting on the date of the General Election just days before the Prime Minister called it.

Craig Williams, who served as Mr Sunak’s parliamentary private secretary and is the Conservative candidate for Montgomeryshire and Glyndwr, apologised for placing the bet but refused to say whether he did so on the basis of insider information.

The Conservative former MP, who represented Montgomery until the election was called, is alleged to have placed a £100 bet on a July polling day some three days before Mr Sunak named the date as July 4, at a Ladbrokes in his constituency.

Mr Williams told the BBC: “I clearly made a huge error of judgment, that’s for sure and I apologise.”

He said on Wednesday that he was facing a Gambling Commission investigation, writing on social media that he would “fully co-operate” with “some routine inquiries”.

Asked repeatedly whether he had had insider information, Mr Williams told the broadcaster on Thursday: “I will not be expanding on that statement because it’s an independent process.

“The Gambling Commission are looking at it now.”

As a parliamentary aide, Mr Williams acted as a go-between for the Prime Minister and his MPs, and is understood to be a trusted member of Mr Sunak’s team.

The Guardian reported that the bet would have led to a payout of £500 based on 5/1 odds.

Foreign Secretary Lord Cameron condemned Mr Williams’s move.

He told BBC Breakfast: “His situation, having made this clearly very foolish decision, is that he’s being investigated by the Gambling Commission, and they have considerable powers in terms of what the consequences could be.”

The Cabinet minister said Mr Williams is already standing as a Conservative candidate and “I don’t think we can change that”, adding Mr Williams “will have to face the consequences” of the Gambling Commission probe.

The Liberal Democrats called for a Cabinet Office inquiry to look into what Mr Williams knew at the time of his bet, with the probe to be given access to any communications between Mr Sunak and his No 10 staff.

Lib Dem leader Sir Ed Davey told broadcasters in Kent: “If someone knows the outcome of something, it seems to me morally questionable for them to put a bet on it if they know the result of that outcome.”

His deputy Daisy Cooper said: “Rishi Sunak must stop being so weak and call a Cabinet Office inquiry into this latest scandal.

“This inquiry is needed to get to the bottom of who knew what when, and uncover whether Craig Williams knew the election date at the time the bet was placed.

“The Conservative Party has been mired in endless sleaze and scandal for years and the British people are sick to their back teeth with it.

“People across the country want a change that’s why they’re backing their local Liberal Democrat champion to kick these Conservatives out of office.”

Rhun ap Iorwerth, leader of Welsh party Plaid Cymru, said there were now “very serious questions” surrounding Mr Williams continuing to stand in the election.

Speaking to the press following his party’s manifesto launch, he said: “I don’t know how quickly that investigation will be concluded, but clearly there are very, very serious questions about the propriety of him remaining as a candidate at all in this election.”