John Gill, 36, was convicted last year after a jury heard he pleasured himself at the Clear Vision opticians, in Knottingley when the girl went to collect some contact lenses.

Sentencing him at Leeds Crown Court today, Judge Rodney Grant described the offence as a "considerable breach of trust" which left the girl distressed.

Gill, of Hunters Way, Selby, North Yorkshire, was found guilty of engaging in sexual activity in the presence of a child following a trial in November.

He was also convicted of outraging public decency after two women, who were with their children at the time, saw him masturbating while he was sitting in a chair in the window of his shop in 2007.

Sentencing him today, Judge Grant said: "(The offence) is a serious matter. It involved you masturbating in your shop in the presence of a 14-year-old girl, which caused her considerable distress."

The judge continued: "The offence of engaging in sexual activity in the presence of a child was, in my view, one which was serious. It was aggravated, in my view, by the fact that you betrayed the trust that (the girl) placed in you.

"She had been a patient of yours for some considerable time and she trusted you to the extent that when she received a telephone call to tell her that her contact lenses were available for collection, she ran down to your shop and it was while she was there you committed the offence for which you were found guilty.

"That, for a young child, is a considerable breach of trust. She was entitled to be protected by you and not subjected to the behaviour that you indulged in."

Judge Grant said: "I've come to the conclusion that the only proper sentence in your case is one of immediate imprisonment."

Bespectacled Gill, who stood in the dock wearing a suit and tie, showed no emotion as he was sentenced to 18 months in prison for the offence of engaging in sexual activity in the presence of a child and a consecutive three-month sentence for outraging public decency.