Cambridgeshire's deputy crime commissioner has quit his post after being accused of "deceiving" a woman "into a sexual relationship".
The allegations, reported by Channel 4 and the Guardian, relate to Andy Coles' work as an undercover police officer in the 1990s.
The matter has been referred to the Independent Police Complaints Commission (IPCC).
Mr Coles said he was resigning because the reports were affecting his duties.
The woman was a 19-year-old political activist at the time of the alleged relationship with Mr Coles.
Mr Coles, who worked for the Metropolitan Police at the time of the alleged relationship, said: "There have been news reports over the weekend about which I am unable to comment.
"This coverage is significantly impacting on my ability to carry out my duties as deputy police and crime commissioner.
"I have therefore today tendered my resignation with immediate effect, which Police and Crime Commissioner Jason Ablewhite has accepted.
"I am aware that the allegations have been referred directly to the IPCC."
Mr Ablewhite said he would not make any comment about the case as it had been referred to the IPCC.