The men were rushed to London's Northwick Park Hospital on Monday. The hospital's intensive care director Ganesh Suntharalingam said all were in a serious condition and receiving "close monitoring and appropriate treatment".

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.

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."

Parexel, which was running the trial, said it had followed guidelines and such cases were extremely rare.

Professor Herman Scholtz, from Parexel, said that the "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."

The incident once again proves just how valuable the results from animal tests really are.