Soham murderer Ian Huntley is being treated in hospital for a drug overdose after being found unconscious in his cell at Wakefield Prison.
He was found in the early hours in the healthcare wing by staff who attempted resuscitation, the Prison Service said.
It has commissioned a review of the "management strategy" for Huntley, who has taken an overdose before.
The 32-year-old received two life terms for the 2002 murders of Jessica Chapman and Holly Wells in Soham, Cambs.
Huntley is receiving treatment at Pinderfields Hospital in Wakefield, where two police officers are guarding the intensive care unit.
The BBC's Home Affairs correspondent Margaret Gilmore said prison sources had confirmed that Huntley was on liquid medication.
They are now investigating how he came to be in possession of what they assume were pills.
The source also said that although Huntley is not expected out of hospital imminently, his condition is not thought to be life-threatening.
The Prison Service said the review of how Huntley is handled would be carried out by the head of its Standards Audit Unit, Rob Kellett.
The Prison Officers' Association (POA) said Huntley had been taken off suicide watch three weeks ago and located in the jail's healthcare centre with other inmates on medication.
POA assistant general secretary Glyn Travis said: "We're not sure what type of medication he has taken but obviously the situation is quite critical and is being investigated by the Prison Service today."
He said the POA had long campaigned for medication in prison to be given in liquid form only.
"But because of the cost of that you still prescribe medication in tablet form which is easily concealed," he added.
Police officers are guarding the hospital's intensive care unit
In June 2003, while awaiting trial for the murder of the two 10-year-old schoolgirls, the former school caretaker saved up 29 anti-depressant pills in a box of teabags and was found suffering a fit on the floor of his cell at Woodhill Prison, near Milton Keynes.
An official report into that suicide attempt uncovered a number of "serious systems failures".
Then Home Office minister Paul Goggins said the internal report revealed a "completely unacceptable situation".
He said: "Procedures for dispensing medication to Mr Huntley fell well short of acceptable standards."
The system officers used to search Huntley's belongings was "predictable" and "did not include the items stored outside of his cell", added the internal Prison Service report.
The document made 15 recommendations, including a review of the jail's cell search strategy and an independent review of security intelligence in the prison, including CCTV.
In September 2005, High Court judge Mr Justice Moses ruled that Huntley, originally from Grimsby, North East Lincolnshire, must serve at least 40 years in prison.
He said the killings did not meet the criteria for a "whole-life tariff", but the 40-year term offered "little or no hope" of his release.
In the same month Huntley was attacked with boiling water by a fellow inmate at Wakefield Prison, which houses some of the UK's most dangerous criminals, including Charles Bronson, dubbed Britain's most violent prisoner.
In January 2004, serial killer Harold Shipman hanged himself in his cell at Wakefield Prison, using a ligature made from bed sheets.
Although the Prison Ombudsman's report said the former GP's death could not have been prevented, it said procedures for dealing with at-risk prisoners needed to be reviewed.