The treatment involved using an influenza virus - one of the most infectious viruses known to man, it was said.
An inquiry is expected, to investigate what safety controls were in place to prevent the spread of infection and exactly what experiments took place at the Hammersmith, one of Britain's top teaching hospitals.
It is claimed that doctors involved in the joint British-Danish research project did not keep their patients in isolation for the three days required.
A spokesman for the Hammersmith Hospitals Trust said: "The trust can confirm that a senior member of the academic clinical staff has been suspended pending an independent inquiry.
"The allegations relate to the possible use of gene therapy, and, because of the complex nature of the matters under investigation and the fact that overseas patients are involved, the inquiry is being set up by the London regional office of the NHS executive."