Simon Hirst, 46, of Observer Mews, Troy Road, was jailed for two years after a trial at Southwark Crown Court last week.

The divorced father-of-two spent four months talking online to someone he believed was a 13-year-old girl but who was actually an undercover police officer.

In one chat he told her: “You look sexy for a young person” while in another he asked her to perform a sex act.

He denied three offences, claiming he could tell by the use of language that he was talking to an adult who he believed was acting out an online fantasy of being a teenager.

During cross-examination, prosecutor Nick Mather asked what his reaction was when the “youngster” told him she was 13.

He replied: “I thought she was just making it up, fantasy.”

He added: “Everyone I see on there is an adult. We talk in adult fashion. There was no reason to believe she was not an adult.”

Hirst said she would have had to have confirmed her adult status to the “webmaster” and added: “My 16-year-old daughter, when she was 12 or 13, had her first computer, we would sit next to her and monitor what she was doing and the language she uses was totally different to what this person was using.”

The barrister asked why “alarm bells did not ring” when the so-called youngster sent him a picture of a young girl.

He replied: “It was part of her fantasy, not mine.”

Hirst, who is the boss of a Leeds-based arcade gaming firm, told the court he believed he was talking to a shopkeeper on a rota system because of the timing of the chats.

He arranged to meet her in London while on business and sent his 23-year-old girlfriend shopping while he sneaked off to the rendezvous.

He told the court during the trial: “I was just going to meet her for a cup of coffee. I didn’t think she was going to turn up. I was expecting a 30-year-old shopkeeper.”

He added that he would have had a heart attack had a teenager turned up to meet him

During the four months, he sent her explicit e-mails as well as a picture of himself indulging in a sexual act before he decided to use a trip to the capital as a chance to meet her.

Jurors took 22 minutes to find him guilty of trying to meet a child after sexual grooming, trying to incite a child to engage in sexual activity and attempting to cause a child to watch a sex act.

Judge Michael Gledhill also ordered that Hirst’s name be added to the sex offenders’ register for 10 years.