Former deputy prime minister Sir Oliver Dowden has become the most senior ally of Rishi Sunak to be interviewed in the official investigation into betting on the date of the general election, Sky News understands.

He follows Mr Sunak’s former parliamentary aide Craig Williams and former Downing Street chief of staff Liam Booth-Smith in being questioned by the Gambling Commission, the statutory body that regulates betting in the UK.

Mr Sunak has not yet been interviewed, Sky News understands, though “numerous people” including Conservative Party officials have been.

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A source close to Sir Oliver said the former senior cabinet minister is not and was never under investigation himself.

It is understood Sir Oliver spoke to the police to assist with their inquiries as part of their investigation into others. This is said to have taken place in early summertime and the officers involved were part of the Gambling Commission.

The inquiry – launched in June – is set to continue for another three to six months.

News of Sir Oliver’s interview, along with the seizing of “hundreds of documents” from Tory HQ by the commission, has emerged on the day the Conservative Party Conference opened in Birmingham.

Ironically, the Gambling Commission’s head office, on the fourth floor of Victoria Square House, Victoria Square, Birmingham, is just a half-mile, 10-minute walk from the ICC, where the Tory conference is taking place.

Sir Oliver was knighted and Mr Booth-Smith was awarded a peerage in the former prime minister’s dissolution honours, announced less than an hour before the polls closed on 4 July.

The commission is investigating whether bets were placed on a July election by people with inside knowledge – in breach of gambling rules – in the days leading up to Mr Sunak’s shock announcement of the election date on 22 May.

A source told Sky News: “The general election betting investigation is still ongoing. Hundreds of documents have been seized by the Gambling Commission from CCHQ.

“The Gambling Commission has also employed more ex-police as investigators to take the case forward. It’s expected the case will continue for three to six months.”

Asked if Mr Sunak has been interviewed, the source said: “I don’t believe so. Numerous people have been interviewed, in and out of CCHQ.

“Gambling Commission investigators have made numerous visits to CCHQ. Oliver Dowden was interviewed.”

What is the election betting scandal?

The election date betting scandal began in June when Craig Williams, formerly MP for Montgomeryshire and Mr Sunak’s parliamentary private secretary, admitted he was being investigated by the Gambling Commission.

Mr Williams had placed a £100 bet on a July election at Ladbrokes in his constituency just days before Mr Sunak announced on 22 May that the election would be held on 4 July. Based on odds at the time, he would have won £500.

“I put a flutter on the General Election some weeks ago,” he said in a post on X on 13 June. “This has resulted in some routine inquiries and I confirm I will fully co-operate with these.

“I don’t want it to be a distraction from the campaign. I should have thought through how it looks.”

Mr Williams, who admitted he had made a “huge error”, was dropped by the Tories as their candidate in the new seat of Montgomeryshire and Glyndwr but remained on the ballot paper, but was defeated, coming third behind Labour and Reform UK.

As the Gambling Commission proceeded, Tony Lee, the party’s director of campaigns, and his wife Laura Saunders, who was Tory candidate for Bristol North West, were placed under investigation.

In a statement on the day news of the investigation was first reported, Saunders said she would be “cooperating with the Gambling Commission”, while Lee took a leave of absence from his role.

Then Nick Mason, the party’s chief data officer, became the fourth Conservative candidate or official to be investigated. He took a leave of absence and denied any wrongdoing.

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In a bizarre twist, a Labour candidate in the election, Kevin Craig, was then suspended by his party after betting against himself and the Gambling Commission launched an investigation into him.

Mr Craig, candidate in Central Suffolk and North Ipswich, posted on X that he had “enjoyed the odd bet for fun” throughout his life.

“A few weeks ago when I thought I would never win this seat I put a bet on the Tories to win here with the intention of giving any winnings to local charities,” he said.

“While I did not place this bet with any prior knowledge of the outcome, this was a huge mistake, for which I apologise unreservedly.”

Then on 27 June Sky News revealed that Mr Booth-Smith, then Mr Sunak’s most senior adviser in Downing Street, had been interviewed by senior Gambling Commission officials and questioned about who knew about the timing of the election.

Sources emphasised, however, that Mr Booth-Smith was not a suspect and was interviewed as a witness and was “asked for help”.

Sky News has approached Mr Dowden and the Conservative Party for comment.