Huntley (33) was released from hospital and returned to HMP Wakefield, where he is serving a life sentence. The incident on Friday was the third occasion on which Huntley managed to get hold of enough drugs to overdose.

Ex-school caretaker Huntley was found guilty of the 2002 murders of Jessica Chapman and Holly Wells, both 10. A prison service spokeswoman said an investigation was under way into events at the prison on Friday afternoon.

Huntley's condition was not thought to be life-threatening.

It is understood he did not lose consciousness and presented himself to prison staff. He was taken to hospital for treatment where he was deemed well enough to return to jail.

Huntley is normally held in the healthcare section of the high-security jail, which is not the same as segregation.

Editor of the Prisons HandbookMark Leech said: "This is a dreadful story for the Prison Service and a huge embarrassment for them, particularly as Huntley is the highest possible security category.

"It is going to require some heads to be banged together to sort this out . . . they really must get to grips with it because this is not the first time, by any means, that Huntley has tried to take his own life. If necessary any medication should be given by injection."