A "dangerous" teenager who stabbed a police officer four times has had his indefinite jail term overturned by top judges.

Alastair Gregson, 19, launched the vicious knife attack on Croydon PC Nigel Albuery as the officer attempted to search him in the street.

Gregson, of Whitfield Avenue, Purley, was caged indefinitely for public protection - which is almost identical to a life sentence - at Kingston-upon-Thames Crown Court last August, after he admitted wounding with intent.

But judges sitting at London's Criminal Appeal Court on Tuesday replaced the open-ended sentence with a 14-year extended jail term - which will see the teenager serve a 12-year custodial term, plus two years on licence in the community.

The court heard Gregson was on bail for attacking his ex-girlfriend when he stabbed PC Albuery in May last year in Bute Road, Waddon.

He was with a group of youths in the area when the officer and a colleague - who were both in plain clothes - approached and spoke to them.

When PC Albuery attempted to search Gregson, the teenager tried to run away before stabbing the officer four times, in his shoulder, back and arms.

Five days before the incident, Gregson hit his ex-girlfriend, Samantha Kerr, with a brick following a heated argument in the street after he saw her with her new boyfriend.

That attack, for which he admitted assault causing actual bodily harm, came just two days after he pleaded guilty to a common assault on Miss Kerr.

Gregson's lawyers accepted he was "dangerous", but argued the crown court judge was wrong not to consider imposing an extended sentence before jailing him indefinitely.

Allowing the appeal, Mrs Justice Dobbs said the offences were "very serious", but that an extended jail term was the right punishment.

The appeal judge, sitting with Sir John Thomas and Mr Justice Underhill, added: "We take the view that this is a case in which an extended sentence can properly be imposed and that, what has been termed the 'nuclear option' is not necessary."