Councillor Marilyn Ashton, Harrow Council's portfolio holder for planning, development and enterprise, is being investigated by the Standards Board for England.
But the watchdog, which investigates and advises on the behaviour of members, will not revealed the circumstances of the complaint against the Conservative member for Stanmore Park.
The authority's ad-hoc assessment committee met in private on July 3 to consider two allegations. Its three members decided to refer part of the first complaint to the standards board and said "no further action" was needed on the second.
It is not known if both of the complaints concerned Mrs Ashton or just one.
A spokeswoman for the Standards Board for England said: "We received a request from Harrow's assessment sub-committee on July 11 to investigate an allegation about Councillor Marilyn Ashton.
"We agreed to investigate it on August 12 and the investigation is currently ongoing. While the investigation is underway we can't confirm the details of the allegation.
"The fact that an allegation has been referred for investigation doesn't indicate any judgement on whether the member has breached the Code of Conduct. That could only be established once the investigation is complete.
"We can usually confirm the outcome of an investigation a couple of working days after the final report has been issued."
The assessment committee - chaired by a member of the public and featuring two councillors from different parties - met most recently on September 25 to consider a three new confidential complaints.
Hugh Peart, director of Legal and Governance Services and, as monitoring officer, the person responsible for complaints, said:
"We have received an official complaint from a member of the public regarding Councillor Marilyn Ashton at Harrow Council.
"After an initial assessment, this matter has now been referred to the Standards Board for England and Wales which is how we are required to deal with complaints against senior councillors.
"Harrow Council is committed to treating complaints from members of the public extremely seriously and will take the appropriate action once the complaint has been received.
"It would be inappropriate to comment on the details of this case as it could be detrimental to the ongoing investigation."