Emergency locksmiths changed hundreds of locks at Belmarsh prison in London after it emerged that a prison officer took keys out of the building by mistake.

The jail holds murderers and rapists along with people who are detained without trial because they are suspected of being international terrorists.

The Prison Service has begun an investigation into how the incident happened.

Security breach

A Prison Service spokesman said: "We can confirm a partial re-lock of HMP Belmarsh has been ordered after it was discovered that a member of staff had taken a set of keys out of the prison."

Prison Officers' Association national chairman Andy Darken said: "Quite rightly, you should not take your keys home because if anybody got hold of them it could lead to escapes.

"It is a real breach of security - but still people can be forgetful.

"It's surprising that a prison like Belmarsh with the high technology we have today lets people take keys through the gates without an alarm sounding.

"Many prisons these days have alarms on the keys so if you try to get out the bells go off."

Belmarsh's most infamous current inmate is Great Train Robber Ronnie Biggs.