A woman who used a schoolboy less than half her age as her “sexual plaything” has been jailed by leading judges who overturned her “unduly lenient” suspended sentence.

Three judges at the Court of Appeal in London replaced the non-custodial term with three-and-a-half years immediate imprisonment in the case of grandmother Marie McCarthy, 62, from Liverpool, who committed sexual offences against a 15-year-old 30 years ago.

The judges agreed with argument presented to them by Solicitor General Robert Buckland that a sentence of 21 months, suspended for two years, which was handed out to McCarthy at Liverpool Crown Court in October, was “unjustified”.

McCarthy, who was aged 33 when the offences took place over five weeks in the summer of 1986, had pleaded guilty to four counts of indecent assault.

Lord Justice Davis, announcing the court’s decision today to quash the suspended sentence, said her conduct was “sustained”.

There were “numerous aggravating factors”, and the court had reached the conclusion that the suspended sentence was unduly lenient – the “appropriate” sentence was one of “42 months immediate custody”.

Lord Justice Davis said there was “very powerful personal mitigation” in her case. A psychiatric report described her as “extremely vulnerable”, and she also had “significant health issues”.

The offences occurred, he said, “during a time when her own personal life was in particular turmoil”.

He said McCarthy was a woman of previous good character, and she had not committed any further offences.

Lord Justice Davis said that at the time of the offences her victim was “relatively immature and a virgin”. He has expressed his “bitterness and anger” at what happened to him.

The sentencing judge told McCarthy that what she had done was “shameful and grotesque”, and said: “You used him as your sexual plaything.”

After the ruling by the appeal judges, Mr Buckland said: “I referred the case to the Court of Appeal as I felt the original sentence failed to fully reflect the seriousness of the crime, which involved Marie McCarthy grooming and indecently assaulting a person who was significantly younger than herself many times over a period of five weeks.

“I hope this immediate prison term sends a strong message that crimes of this nature will not be tolerated and that male and female victims and perpetrators will be treated equally before the law.”