An allegation of misconduct is being investigated against a police force's chief constable by the Independent Police Complaints Commission.
Sussex Police Authority (SPA) voluntarily referred the allegation against Sussex Chief Constable Martin Richards to the IPCC last month.
Details of the incident have not been released by Sussex Police but it is believed Mr Richards is being investigated over allegations he used "undue influence" on a force operation, according to the Brighton Argus newspaper.
The complaint was made about him by a member of staff using the force's anonymous internal reporting system, the paper said.
A Sussex Police spokeswoman said that given the seniority of the office of Chief Constable Sussex Police believes it is paramount that any allegations about him are subject to the independent scrutiny of the IPCC in order to maintain the public's confidence in the outcome of any investigation.
She said: "We cannot disclose the nature of the allegations at this time, however, due to the risk of an impact on an on-going operational matter. We will provide more details when there is no longer a potential impact on operational matters or when the IPCC concludes their investigation."
She said the force welcomed their scrutiny and would provide any support required to the investigation.
Mr Richards said: "Given the senior position I hold and the trust placed in me by the people of Sussex and my colleagues, it is absolutely right that the Sussex Police Authority has voluntarily referred this matter to the IPCC for independent scrutiny. I will cooperate fully with the investigation to help them establish the facts.
"While the IPCC investigation is ongoing it will be business as usual. There will be no impact on the service provided to the public or the delivery of operational policing, which is our foremost priority."
Sussex Police said the decision to investigate Mr Richards is a neutral act and does not indicate an assumption of wrong-doing.