A double murderer attacked an officer in a maximum security prison in Worcestershire, holding a knife to his throat and shouting: "I'm going to slit your f****** throat", a court has heard.

Daniel Sonnex attacked Richard Stringfellow at HMP Long Lartin, where he was serving life with a minimum of 40 years for two murders, Reading Crown Court heard.

Sonnex, 27, denies a charge of false imprisonment of Mr Stringfellow on June 19, 2010, in which he is said to have "assaulted and unlawfully and injuriously imprisoned and detained" him against his will, as well as threatening to kill him.

The killer, formerly of Deptford, south-east London, is now being held at high-security psychiatric hospital Broadmoor, and appeared in the dock flanked by five male nurses. He also wore sunglasses in court due to a medical condition.

Opening the case, prosecutor Peter Grice told the jury that Sonnex followed Mr Stringfellow as he walked through the wing, jumping on his back, and holding a vegetable knife to his throat, threatening to kill him.

Mr Grice told the court that in June 2010, Sonnex was serving life with a minimum term of 40 years at HMP Long Lartin near Evesham, Worcestershire, for two murders, and also had previous convictions for wounding with intent and robbery.

"On June 19, 2010, prison staff became aware that shelves had been taken from a fridge that prisoners used, also the back element had been removed from a fridge," Mr Grice told the court.

"There came a stage just after 2.30pm in the afternoon where a prison officer, Richard Stringfellow, left the wing, made his way outside to the exercise yard, and as he did so he saw the shelves and the element, strapped together using bedding, and picked them up."

The court heard Mr Stringfellow picked up the shelves, and returned to the wing, which he began to walk through. "As he did so he was followed by the defendant," Mr Grice said. "What happened next is the defendant jumped on Mr Stringfellow's back.

"He put his arms around his neck and he put a knife, a vegetable knife, to his throat. As he did he shouted at Mr Stringfellow, 'I have got a knife, I'm going to slit your f****** throat'."