Two men were continuing to fight for their lives this morning after falling desperately ill when taking part in a drug trial.
They were in a critical condition in hospital while four others were "serious" but showing some signs of improvement.
They were admitted from an independent medical research unit on the North London hospital after taking part in the trial.
The drug, known as TGN1412, is made by the pharmaceutical company TeGenero AG, based in Wurzburg, Germany.
It is intended to fight leukaemia, rheumatoid arthritis and multiple sclerosis.
The company said these were "shocking developments" in the testing of a new medicine which showed no signs of any safety problems in previous testing.
Thomas Hanke, chief scientific officer, said: "Our first concern right now is for the patients and families and that they get all treatment possible. We are confident with the excellent treatment at this hospital and have made ourselves available to answer any questions from the doctor about the drug."
Scotland Yard confirmed it was talking to the UK medicines watchdog, the Medicines and Healthcare products Regulatory Agency (MHRA).
A police spokesman said: "We are liaising with the MHRA to establish the circumstances around the clinical trial of the drug."
The MHRA immediately suspended the trial when the men fell ill and sent inspectors to the research unit to carry out an investigation.
Parexel, the clinical research company running the trial, said it had operated within regulatory guidelines and that such adverse reactions to drugs were extremely rare.
Eight men were taking part in the trial but two were given a placebo and were unharmed.
Ganesh Suntharalingam, the hospital's clinical director of intensive care, said two patients remained critical and four were serious but showing some signs of improvement.
"We have given some treatment aimed at the immune system and the patients are continuing to receive a whole range of treatments that the critical care service offers."
One victim was named as student Ryan Flanagan, 21, of Highbury, north London.
His family were told he could not breathe and his head and neck had swollen to three times normal size.
Family friend Sarah Brown, 27, said: "Ryan was a healthy young man and he saw the t rial advertised on the internet.
"He is at college and was doing it to make a bit of extra money.
"He told us he would be paid £2,000 and did not think there would be any problems.
"His mother got a call to say his head and neck were swelling up and his legs were purple."
Health Secretary Patricia Hewitt said: "Obviously our thoughts are with those young people and with their families.
"I know that everything is being done at the hospital to look after them."