UK police announced on Friday that a probe into betting on the date of the last general election had ended, saying the bar for misconduct had not been met.

London's Metropolitan Police said in late June it would investigate a "small number" of bets placed on the timing of last month's elections, as part of a joint probe with the gambling watchdog.

The scandal overshadowed the latter stages of campaigning ahead of the July 5 poll, after it emerged senior Conservative officials and one of then-prime minister Rishi Sunak's aides had placed bets shortly before he announced the date.

The Met also arrested one of its officers on suspicion of misconduct in public office over election betting claims, while six other officers were identified as having placed bets on its date.

The Gambling Commission launched an investigation into whether any broke laws on "cheating" while the Met opened a separate inquiry into whether other criminal offences had been committed.

While political bets are allowed in the UK, using insider knowledge to make them is against the law.

On Friday, Britain's biggest police force announced its criminal investigation had concluded with no action being taken, including against the officer who had been arrested.

The Met said the "high bar" for proving misconduct in public office had not been met, following advice from the country's prosecution.

The Gambling Commission investigation remains ongoing and could still result in criminal charges.

Meanwhile the Met's internal watchdog unit is continuing to investigate the seven officers alleged to have placed bets on the election date.

"These allegations caused a significant dent in public confidence during the election campaign and it was right that they were investigated to explore all possible offences," Detective Superintendent Katherine Goodwin said in a statement. (AFP)