Conservative Campaign Headquarters on Thursday said that Tony Lee "took a leave of absence" on Wednesday, and the Gambling Commission was looking into "a number of individuals".

The regulator made no mention of identities but the BBC reported that Lee and his wife Laura Saunders, who is standing as a candidate in the July 4 poll, were under investigation.

The commission is also looking at claims that another would-be MP, Craig Williams, who served as Sunak's ministerial aide, made a £100 ($127) stake on the date before it was called.

London's Metropolitan Police (the Met) has also said one of Sunak's security detail -- a police officer -- was arrested for allegedly placing a bet on the date of the poll.

Political bets are allowed in the UK, including on the date of the election, but using insider knowledge to do so is against the law.

The formal inquiry heaps further misery on Sunak, whose party has trailed Labour by about 20 points in the polls for nearly two years, making it likely that it will be dumped out of office after 14 years in power.

On the campaign trail, senior minister Michael Gove admitted to reporters that the situation "doesn't look great", even if he could not comment on the specifics.

But earlier he said that the "broad principle" of using inside information to place bets was "reprehensible".

Labour leader Keir Starmer, tipped to succeed Sunak in Downing Street, urged the prime minister to withdraw his support for those allegedly involved.

"It's astonishing that we're in this place... The government, Rishi Sunak, just needs to take action. He needs to account for exactly who knew what," he said.

Labour's campaign co-ordinator Pat McFadden separately wrote to Sunak and said the claims were "a pattern of behaviour" in the Tories focused on making "a quick profit".

Ed Davey, who leads the smaller Liberal Democrats, said the allegations smacked of "corruption". "It needs to have a heavy hand from the top," he added.

Wipe-out?
 

Sunak announced the date of the election on May 22, in a rain-soaked statement outside Downing Street, taking his party by surprise as he still had six months to call a vote.

Critics lambasted him for not using an umbrella and the campaign has hardly shifted the dial in his favour since then.

One reporter asked him if he was the captain of a sinking ship at a photocall in Belfast, near where the doomed Titanic was built.

This week, he was ignored by a flock of sheep as he tried to feed them in southwest England while personally his ratings have suffered after he left early from a D-Day 80th anniversary commemoration event.

Two polls published on Wednesday predicted a record win for Labour, eclipsing even the landslide victory for former leader Tony Blair in 1997, and a historic drubbing for the Tories.

Pollsters YouGov said the Conservatives -- the party of Margaret Thatcher and Winston Churchill -- could slump to their "lowest seat tally in the party's almost 200-year history".

Sunak could even become the first sitting prime minister to lose his seat, according to a Savanta survey.

The Met said its officer, a member of the Royalty and Specialist Protection Command, was held on Monday on suspicion of misconduct in a public office, then released on bail pending further enquiries.

The force has referred itself to the Independent Office for Police Conduct.

The allegations against Williams came to light last week. He is alleged to have placed a bet on a July date for the election three days before Sunak called the vote.

Sunak and other party leaders, including Labour's Keir Starmer, take part in another pre-election television grilling later on Thursday.