A director of finance with West Midlands Police has quit his £121,000-a-year post for charging £200 to his official expenses account for an unauthorised hotel stay with a female colleague.
Married Derek Smith quit as director of finance and resources post before an official inquiry could be completed.
He had worked for West Midlands Police for 20 years, helping lead the force through a £750 million cuts plan and had advised the Government on charging for police services.
He has now offered to pay the money back.
An investigation was today ongoing by the force's professional standards department in to the conduct of the woman, a member of the back office staff.
It has emerged that the pair checked into a Manchester hotel, and Mr Smith charged the full £200 - which is believed to include accommodation, drinks, food and travel expenses - to the force.
West Midlands Police spokesman Lee Page said the force's professional standards unit was "made aware of an allegation of misconduct against two members of police staff" in November.
He said: "The allegation concerns the use of police funds in connection with expenses, travel and accommodation.
"One individual has left the force. The second individual concerned continues to be investigated."
He said Mr Smith, the force and the police authority had all agreed his position had become "untenable".
He defended the decision not to take the investigation any further, saying: "Because of the senior position held in this case if a decision was taken to bring misconduct proceedings, this would be carried out by senior representatives from another force.