Three serving policemen and a retired police officer have been arrested as part of an inquiry into potential fraudulent activity at the Police Federation of England and Wales.

Surrey Police are investigating an alleged transfer of about £1m to a charitable account in August last year.

One of those arrested is understood to be Will Riches, who quit as federation vice-chairman earlier this week.

Scotland Yard has said two of the men are Met police officers on secondment.

The Police Federation says it is cooperating with the enquiry.

The organisation represents thousands of officers of constable, sergeant and inspector rank.

"We are carrying out a detailed and thorough investigation into allegations of fraudulent activity involving significant amounts of money," said Detective Superintendent Karen Mizzi from Surrey police.

The four men, two in their 40s and two in their 50s, remain in custody while inquiries continue, Surrey Police said.

The federation's chairman, Steve White, general secretary, Andy Fittes, and treasurer, Martyn Mordecai, are understood not to be among those arrested, the BBC's Danny Shaw says.

Specialist officers from Surrey Police's economic crimes unit were involved in the probe, Det Supt Mizzi said.

The Independent Police Complaints Commission has also been notified.

The Police Federation of England and Wales first raised concerns with Surrey police - the federation's local force - on Tuesday.

The federation then made a formal allegation relating to accounts held by the Federation's Constables' Central Committee on Wednesday.

Det Supt Mizzi said Surrey police had contacted the relevant police forces to inform them of the arrest of their officers.

The force would also be contacting the Charities Commission, she said.

Anyone with information is asked to call the Serious and Organised Crime Team on 101, quoting Operation Rutland.