A high-risk sex offender will be supervised in the community for three years when he is released from prison after a landmark Court of Appeal ruling.

Merrick Seaman, now 30, was jailed for eight years for subjecting a five-year-old girl to a serious sexual assault in July 2010.

During a complex sentencing hearing in 2011, the former Chief Justice Richard Ground ruled that Seaman should be imprisoned at Westgate rather than be made the subject of a Hospital Order under the Mental Health Act. However, he did not consider what provisions should be made after his release.

As a result, prosecutors launched an appeal against the original sentence to ensure Seaman would be monitored after he was freed.

Yesterday, the Court of Appeal allowed the Department for Public Prosecutions (DPP) to make that appeal, even though more than four years had elapsed since the sentencing and appeals usually have to be lodged within 28 days.

It is believed to be the first time that the Court of Appeal has allowed the DPP to appeal against a sentence “out of time”.

President of the Court of Appeal, Sir Scott Baker, ordered that Seaman would be supervised in the community for at least three years after his release.

At present, the earliest point at which he could be released on parole would be December 2017; however the eight-year sentence does not end until 2019.

Describing the DPP’s application as “unusual”, prosecutor Cindy Clarke said: “It is quite clear the issue of a supervision period was never raised.

“The Chief Justice was never asked to make the order, he was thinking more in terms of Hospital Order or imprisonment.”

Seaman’s lawyer, Ken Savoury, told the Court of Appeal his client had been receiving psychological treatment since he was very young. He conceded the supervision order was in the public and his client’s best interests, and that his client’s parents would not be able to look after him when he was released.

In allowing the DPP’s appeal against sentence Justice Baker said: “It seems to us that it is in the interests of the public but also the defendant in light of his offence, his history of problems controlling his sexual behaviour to impose a period of supervision for three years.

“When he is released he will have to be accommodated somewhere and that is something that needs to be considered carefully by the probation service at the time.”