Three serving policemen have been arrested on suspicion of fraud in an investigation that centres on the organisation which represents rank-and-file officers.
The probe into an allegation of "potential fraudulent activity" was sparked after the Police Federation of England and Wales raised concerns with police on Tuesday.
A day later a formal allegation was made relating to accounts held by the Federation's Constables' Central Committee.
Surrey Police, which is investigating, said the allegation "relates to concerns about the transfer of around £1 million to a charitable account in August 2015".
The force said two men in their 50s and two men in their 40s were arrested on Friday morning on suspicion of fraud under Section Four of the Fraud Act and conspiracy to defraud.
Three of those arrested are serving police officers. All four men remain in police custody while inquiries continue.
Detective Superintendent Karen Mizzi, who is leading the inquiry, said: "We are carrying out a detailed and thorough investigation into allegations of fraudulent activity involving significant amounts of money.
"As the allegation was only reported to us on Wednesday 16 March our inquiries are at an early stage but specialist officers, including our Economic Crime Unit, are investigating to establish whether any offences have occurred."
She added: "The Police Federation reported the matter to us as their headquarters is in Leatherhead and we are their local force but to be clear those arrested are not Surrey Police officers.
"Our Professional Standards Department is currently in liaison with colleagues in relevant forces to inform them of the arrest of their officers.
"Due to the nature of the allegation, and three of those arrested being serving officers, Surrey Police has notified the Independent Police Complaints Commission (IPCC) and will also be in liaison with the Charity Commission going forward."
The Police Federation represents tens of thousands of officers of Constable, Sergeant and Inspector rank.
In a statement on its website, the organisation said it is "cooperating fully" with the inquiry and "gives Surrey Police its utmost support".
It added: "The day-to-day running of the PFEW is not affected and the PFEW remains committed to representing its 122,000 members.
"Until the inquiry is complete, it would be inappropriate to comment further."