Six men remain in intensive care after being taken ill during a clinical drugs trial in north-west London.
The six are being treated at Northwick Park hospital
The healthy volunteers were testing an anti-inflammatory drug at a research unit based at Northwick Park Hospital when they suffered a reaction.
Relatives are with the patients, who suffered multiple organ failure. Two men are said to be critically ill.
An investigation has begun at the unit, run by Parexel, which said it followed recommended guidelines in its trial.
The men were being paid to take part in the early stages of a trial for the drug to treat conditions such as rheumatoid arthritis and leukaemia until they were taken ill on Monday within hours of taking it.
Eight volunteers were involved, but two were given a placebo at the unit which is on Northwick Park Hospital's grounds but is run independently.
The hospital's intensive care director Ganesh Suntharalingam said the patients were admitted very quickly and were receiving "close monitoring and appropriate treatment".
A Northwick Park Hospital spokesman said two were in a critical condition, while the other four were "serious but stable".
Richard Ley, spokesman for the Association of the British Pharmaceutical Industry, said: "This is an absolutely exceptional occurrence. I cannot remember anything comparable."
The Medicines and Healthcare products Regulatory Agency (MHRA) immediately withdrew authorisation for the trial. An international warning has also gone out to prevent it being tested abroad.
Its inspectors will visit the research unit and it is in contact with the local strategic health authority, the Department of Health and police about the cases.
Chief executive officer Professor Kent Woods said: "Our immediate priority has been to ensure that no further patients are harmed.
"We will now undertake an exhaustive investigation to determine the cause and ensure all appropriate actions are taken."
It had approved the trial and the drug had already been tested on animals and in a laboratory.
Volunteers are paid up to £150 a day to take part in clinical trials.
Healthy volunteers are used to test the safety of the drug in "phase one" of the trial before further tests with people who have the condition to determine whether the drugs work.
Parexel, which was running the trial, said it had followed guidelines and such cases were extremely rare.
Professor Herman Scholtz, from Parexel, said the clinical research organisation had followed regulatory, medical and clinical research guidelines during the study.
He said: "When the adverse drug reaction occurred, the Parexel clinical pharmacology medical team responded swiftly to stop the study procedures immediately."
He added: "Such an adverse drug reaction occurs extremely rarely and this is an unfortunate and unusual situation.
"Since our unit is located within the hospital, we have immediate access to world-class medical care and we did everything possible to get the patients treated as quickly as possible."
Have you taken part in a drug trial? How did you find the experience? Would you consider taking part in one? Have you conducted a drug trial? Send us your comments and experiences.