A judge's decision not to jail a deputy headteacher, despite the fact he admitted downloading 7,466 child-porn images, cannot be challenged it has emerged.

Former Wantage County Primary School teacher Phillip Carmichael, 58, was given an absolute discharge at Oxford Crown Court last week after his defence argued the drugs he took for Parkinson's Disease gave him "uncontrollable" sexual urges.

He admitted downloading indecent images and videos, but blamed his actions on the drugs he was taking.

He was said to be suffering from "hypersexuality" as a result of taking Cabergoline and Ropinirole.

The Crown Prosecution Service has now confirmed the type of offences prevents it from challenging Judge Mary Jane Mowat's decision.

Sentencing last Thursday, Judge Mowat said: "This is a wholly exceptional and very distressing case.

"To say he was to blame would be a complete denial of the reality. He was not only an ill man at the time, but a man whose medication can be regarded as wholly responsible for the commitment of these offences."

Carmichael was arrested after police raided his home in Adkin Way, Wantage, in September last year.

He was traced after emailing a website to complain about the quality of the images he had received.

The former teacher's sentence was met with concern from campaigners, including children's charity Kidscape.

Its director Claude Knights said: "It is surprising and disappointing this has been taken fully as a mitigating circumstance."

A CPS spokesman said they were unable to appeal as Carmichael's offences were not included in a statutory instrument determining which cases can be referred to the Attorney General.

A statutory instrument is a form of secondary legislation used to set out exactly how the broad principles of an Act will be applied.

Cathy Olliver, the case prosecutor against Carmichael, said: "We cannot appeal as it is not an offence for which we have the power to appeal."

In October 2007, a child rapist's sentence was doubled after his original two-year jail term was referred to the Attorney General by the CPS.

Keith Fenn, a window cleaner from Blackbird Leys, who admitted raping a 10-year-old girl, had initially been given the "unduly lenient" punishment by Judge Julian Hall.

But the Attorney General, Baroness Scotland, appeared before the Court of Appeal to argue Fenn's sentence was too lenient.