Six men who took part in clinical trials of an anti-inflammatory drug were rushed to intensive care last night.
The trialists, aged between 18 and 40, suffered multiple organ failure after taking the TGN 1412 drug designed to treat conditions such as leukaemia and rheumatoid arthritis.
Two men are believed to be in a critical condition; with a further four left in a serious but stable state following the trial.
The calamitous outcome of the test, which was conducted by Paraxel, has prompted the Medicines and Healthcare Products Regulatory Agency (MHRA) to suspend the trial at Northwick Park hospital in north-west London. The Metropolitan Police have announced they will conduct an investigation.
An international alert has also been issued to ensure the drug does not undergo testing elsewhere.
Professor Kent Woods, from the MHRA, told the BBC that there were several possibilities as to what had gone wrong.
"We need to look at the quality of the drug, has there been some manufacturing problem, has there been some issue of contamination, has there been a dosing error or is this indeed some completely unanticipated side effect of the drug in humans which is specific to humans," he said.
Two men who took a placebo were unharmed.