The scandal is undermining the Conservatives’ efforts to close the gap on the opposition Labour Party’s large poll lead eight days before the national election.

The expanding number of Conservative officials under investigation has resulted in support for the party falling to 18%, down two percentage points, the poll Survation published today showed. The Labour Party was on 41%.

Since the betting allegations first emerged, the conduct of government has become the second most important issue for voters behind the cost of living crisis, the poll found.

The betting scandal intensified further late yesterday after a cabinet minister said he had placed three bets on the date for of the election, but that he was not being investigated by the gambling regulator. A Labour candidate also admitted placing a bet against himself.

Both the Conservatives and Labour yesterday withdrew support for candidates under investigation. Sunak had faced days of criticism for initially failing to withdrew suspend the candidates being investigated by the regulator.

While Sunak said he was “extremely angry” about the scandal, he previously stopped short of withdrawing support so the investigations could finish before any action was taken.

Mel Stride, the work and pensions minister, said today that more candidates could be involved in the scandal and there should be a debate about political betting.

For the poll, Survation interviewed 1,022 adults in Britain by phone between June 21 and 25.