During their relationship Mr. McCarthy also sent sexual text messages to the woman, it was found.
The harassment case against him was later closed.
It is clear that John McCarthy took violent steps for his own sexual gratification and personal gain.
Mr McCarthy also borrowed £3,580 from the woman in a series of payments between June and September 2017, IOPC met.
The IOPC said it had found evidence that Mr McCarthy did not intend to pay back the woman.
It said that the then DC neither declared his relationship with the woman nor the money received from her.
However, Mr McCarthy’s investigation was launched in January 2018 following a referral from the Met Police.
Investigation in March 2020 found that there was a case of gross misconduct against the officer.
Mr McCarthy then resigned from the force on 5 September this year.
In a statement, IOPC regional director Sal Naseem said: “It is clear that John McCarthy took violent steps for his own sexual gratification and personal gain.”
Mr Naseem said Mr McCarthy’s behavior has had a “corrosive” effect on public trust in police.
“His behavior has had a severe and devastating effect on his victim, and a corrosive, lasting effect on the public’s confidence in individual officers and the police service in general,” he said.
A police disciplinary panel ruled that Mr McCarthy would have been dismissed for gross misconduct without notice if he had not resigned earlier.