UK Prime Minister Rishi Sunak faced calls on Thursday to launch an inquiry after a member of his security detail was arrested for allegedly betting on the timing of the general election.

London's Metropolitan Police said it was told by the Gambling Commission that a close protection officer was being investigated over the alleged bets.

The regulator had already been looking into claims that Conservative party candidate Craig Williams, who served as Sunak's ministerial aide, placed a bet on when the election would be held.

A second candidate from Sunak's party, Laura Saunders, is now also under investigation about an alleged bet on the date of the poll, the BBC reported on Wednesday evening.

Saunders is married to the Tories' director of campaigns, Tony Lee. He "took a leave of absence from CCHQ (Conservative Campaign Headquarters) yesterday" (Wednesday), the party said.

CCHQ confirmed earlier that the Gambling Commission had contacted it about "a small number of individuals", without commenting further.

Senior minister Michael Gove meanwhile told reporters on the campaign trail that the fact Saunders was facing an investigation "doesn't look great".

He said he could not comment on the specific allegations but earlier said the "broad principle" of using inside information to place bets was "reprehensible".

Labour leader Keir Starmer, tipped to succeed Sunak in Downing Street, called on 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 that a general election would be held on July 4, taking his own party by surprise as he still had six months to call a vote.

The Tories are expected to be dumped out of office after 14 years in power, with polls consistently putting the main opposition Labour party some 20 points ahead.

Two polls published on Wednesday predicted a record win for Labour, eclipsing even the landslide win 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".

As well as criticism of Sunak's decision to go early, the Tory campaign has been widely criticised for a series of gaffes, including a photocall in Belfast near where the doomed Titanic was built.

Sunak's own already dire personal ratings were also hit after he left an international event to mark the 80th anniversary of D-Day early.

The Met said the officer, a member of the force's Royalty and Specialist Protection Command, was no longer on operational duties.

The officer was taken into custody on Monday on suspicion of misconduct in a public office, then released on bail pending further enquiries.

A referral has been made to the Independent Office for Police Conduct, the watchdog confirmed.

The allegations against Williams came to light last week. He is alleged to have placed a £100 ($127) 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.