Detectives are investigating allegations of fraud involving the Police Federation of England and Wales, the representative body has announced.
Accounts held by one of the federation's national committees are the focus of the investigation by Surrey Police that began on Tuesday.
The body, which represents the majority of officers in the country, said it had requested the force look into the accounts and no arrests have been made.
A federation spokeswoman confirmed separately that its vice chairman, Will Riches, had resigned from his post but refused to give reasons why.
In a statement released on Thursday, the federation said it was co-operating with the investigation into "potential fraudulent activity" surrounding accounts held by its Constables' Central Committee.
The Police Federation of England and Wales has three central committees of elected representatives from the ranks of constable, sergeant and inspector drawn from eight regions.
The federation said: "On Tuesday 15 March, the Police Federation of England and Wales contacted Surrey Police to ask them to investigate potential fraudulent activity, relating to accounts held by the federation's Constables' Central Committee.
"The issue was identified by the PFEW itself and we are fully co-operating with their initial investigation which is being undertaken to determine whether any offences have been committed.
"Until those inquiries are complete it would be inappropriate to comment further."
The spokeswoman said Mr Riches, a constable in the Metropolitan Police, remained on the body's Interim National Board.