Many more people than the leading Tories already identified are being investigated by the gambling regulator in relation to bets on the date of the general election, Sky News understands.

And in a major new development, the watchdog has widened its inquiries to investigate whether people with inside knowledge may have asked a third party to place a bet for them.

Sky News also understands that investigators have written to leading bookmakers asking for details of bets of £20 or more on the election date within days of Rishi Sunak announcing it on May 22.

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The body carrying out the investigation, the Gambling Commission, already takes a close interest in political betting and carries out extra checks on bets by people connected with politics, Sky News has been told.

The gambling industry also regards politicians and those who work closely with them as PEPs, politically exposed persons who are people “entrusted with a prominent public function”.

That’s because these people generally “present a higher risk for potential involvement in bribery and corruption by virtue of their position and influence”, according to the Law Society.

It emerged on Saturday evening Nick Mason, the Tory party’s chief data officer, was the fourth Conservative candidate or official being investigated. He has taken a leave of absence and denies any wrongdoing.

Mr Sunak’s parliamentary private secretary Craig Williams, the Tory candidate for Montgomeryshire and Glyndwr, admitted last week to placing a “flutter” on the date of the election.

Earlier this week, Tony Lee, the party’s director of campaigns, and his wife Laura Saunders, the Tory candidate for Bristol North West, were also placed under investigation.

Mr Sunak’s close protection officer has been arrested for allegedly betting on the timing of the election.

Home Secretary James Cleverly told Sky News’ Sunday Morning with Trevor Phillips he had been told “very, very clearly” to not discuss the investigation.

He added: “I’m not in any way going to defend people who placed bets on that.”

Asked whether there was a wider betting circle, Mr Cleverly said: “That’s not my understanding. My understanding is it’s a small number of individuals.”

He also said he has “no reason to believe” any ministers are involved in betting on the timing of the election.

A Gambling Commission spokesman said: “The Gambling Commission regulates gambling in the interests of consumers and the wider public.

“Currently, the Commission is investigating the possibility of offences concerning the date of the election.

“This is an ongoing investigation, and the Commission cannot provide any further details at this time. We are not confirming or denying the identity of any individuals involved in this investigation.”

A Conservative spokesman told Sky News: “As instructed by the Gambling Commission, we are not permitted to discuss any matters related to any investigation with the subject or any other persons.”

The other candidates for Bristol North West are:

Caroline Gooch, Lib Dems

Darren Jones, Labour

Scarlett O’Connor, Reform UK

Mary Page, Green Party

Ben Smith, SDP

The other candidates for Montgomeryshire and Glyndwr are:

Jeremy Brignell-Thorp, Green Party

Oliver Lewis, Reform Uk

Glyn Preston, Lib Dems

Elwyn Vaughan, Plaid Cymru

Steve Witherden, Labour