A prisoner who left another inmate needing 50 stitches in his face after slashing him with a "vile-looking weapon" has been jailed for eight-and-a-half years.

Michael Birch, of Valley Drive, Gravesend, had only been in Belmarsh Prison a short while when he launched the vicious attack on Lee Moore.

The 31-year-old thug used a blade attached to a piece of plastic to cut the victim across the jaw with a single blow on October 13 last year.

He admitted wounding with intent to do grievous bodily harm shortly before he was due to stand trial at Maidstone Crown Court.

Judge Jeremy Carey told Birch he had decided to take matters into his own hands, although it was not possible to know the full story.

"You got hold of an instrument capable of causing very grave injury, indeed if not death," he said. "It cannot be certain you were the one to fashion that vile-looking weapon, but what is plain is you had time to think about whether you were going to set about Mr Moore or not and decided to use the weapon.

"There was, therefore, premeditation. You thought about it and you could have made the right decision not to go ahead, but went ahead with a purpose.

"You went along the landing to the cell and slashed his face, opening up his jaw, which seeped vast amounts of blood. It must have frightened the life out of him."

Mr Moore, who was serving three years for burglary, was left scarred and traumatised.

Birch was serving 12 months at the category A prison, in south east London and known for its tough regime, for assault causing actual bodily harm.

"You have convictions for violence, but nothing as serious as this," the judge told him.

"Those who continue to offend will be treated severely."

Judge Carey added: "There is precious little mitigation. You are leading a completely wasteful life. You have nothing I have heard of you can say you are proud of.

"You have indulged in class A drugs. You have been a complete pain in the neck for everyone. You have let down the people who love you.

"You have had a miserable life which is going to continue to be miserable in a custodial setting. You are pretty close to dangerous offender category. Any more of that and you will find yourself serving a life sentence."