Two other patients who took part in the drug tests are receiving less organ support as their condition improves and all four are conscious and able to talk to their families, a statement from Northwick Park Hospital in northwest London said.

But two men . . . 21-year-old Ryan Wilson, whose mother is Irish, and a 28-year-old man reported to be an assistant bar manager . . . remain in a critical condition.

Early signs of improvement in the men's condition on Friday have continued, but doctors said because of their "complex and very serious condition" it was too early to give a prognosis.

"In response to the treatment they have had, two patients have improved sufficiently to stop receiving organ support. They do still need high-dependency care and remain under the supervision of the critical care team.

"Two of the other men remain in a serious condition, but are gradually receiving less organ support.

Both these men, and those in highdependency beds, are fully conscious and chatting with their relatives, " the statement said.

All of the men are being given anti-inflammatory treatment following their reaction to the drug, TGN1412.

Dr Ganesh Suntharalingam, director of intensive care at the hospital, said an advisory panel was meeting regularly and developing a more detailed understanding of what happened to the previously healthy men, who fell ill on Monday after taking part in tests for the drug.

TGN1412 was being developed by German firm TeGenero to treat conditions such as rheumatoid arthritis, leukaemia and multiple sclerosis, and was being tested on humans for the first time.

But TeGenero's chief scientific officer, Thomas Hanke, has said the drug was tested on rabbits and monkeys with no "drug-related adverse events".

The medical research company responsible for the drug trial, Parexel, insists it followed correct procedures.