A senior police officer who met an internet date for sex while on duty was acquitted of wilful misconduct yesterday after explaining that he had worn his radio earpiece throughout the encounter.

Masood Khan, 41, a British Transport Police inspector, booked an official police room at Gatwick airport railway station for the 20-minute liaison. Mr Khan, who was responsible for passenger safety across the southeast of England, had contacted the 43-year-old woman for the first time the previous day on the website uniformdating.com.

He was tried for misconduct in a public office but a jury at Southwark Crown Court unanimously found him not guilty after deliberating for just ten minutes. He now faces a police disciplinary inquiry after he admitted in court that he had acted unprofessionally. “It was absolutely the wrong thing to do, morally and professionally,” he said. “I should never have done it.”

Mr Khan, who was in charge of a team of 20 officers, had claimed that he was ready to respond to any emergency because he was wearing his radio earpiece while he had sex. He refused to comment as he left the court.

Judge Nicholas Lorraine-Smith criticised the Crown Prosecution Service at the start of the case for going ahead with the trial. “I don’t think it was necessary for him to be charged,” he said. “It is not as serious as many of these offences. The Crown have called no evidence that there was something he was supposed to be doing but wasn’t doing. As the man in charge he just had to be on the end of a telephone; he didn’t have to be there. This behaviour was disgraceful, let’s not get away from it, but I am curious to see why it was thought necessary for him to be charged.”

He said that he was “firing a shot across the CPS’s bows” for overusing the misconduct charge. “Misconduct is a serious offence and were this defendant to be convicted he would have a criminal offence on his record.”

Mr Khan had been a serving officer for 15 years and as inspector was responsible for rail passenger safety across South London, Kent, the Thames Estuary and the South Coast as far west as Dorset.

Michael Mulkerrins, for the prosecution, said that Mr Khan logged on to uniformdating.com on July 22 last year and made arrangements to meet the woman the next day, when he would be on duty. He was due to work from 9am to 4pm that day but he drove in a marked police car from his office in Victoria, Central London, to Gatwick station before 3pm.

He had telephoned a junior officer to inquire about the availability of a police room at the station, stating that he needed it for “official police business” and giving the name “Michael Plod”. After telephoning the woman, Mr Khan arrived at the station and stood on the concourse in full uniform before meeting up with her.

CCTV footage showed the couple going to the police room on Platform 2 at the station and emerging again 20 minutes later. Mr Khan accepted that they had sex but said that he was listening for any calls. “The new radios we wear in our lapels – I had an earpiece in, on low volume. If there was a call for me I would have answered it and dealt with it.”

Kevin Baumber, counsel for Mr Khan, said that his client’s conduct amounted to “misbehaviour but not criminal behaviour”.

He said: “This is a criminal court, not a moral court. It’s doubtful this case would have been brought if he’d taken an extended lunch break or gone for a game of golf.”

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