The medical staff, from the northern town of Grudziadz, are being investigated over medical trials on as many as 350 homeless and poor people last year, which prosecutors say involved an untried vaccine to the highly-contagious virus.

Authorities claim that the alleged victims received £1-2 to be tested with what they thought was a conventional flu vaccine but, according to investigators, was actually an anti bird-flu drug.

The director of a Grudziadz homeless centre, Mieczyslaw Waclawski, told a Polish newspaper that last year, 21 people from his centre died, a figure well above the average of about eight.

Although authorities have yet to prove a direct link between the deaths and the activities of the medical staff, Poland's health minister, Ewa Kopacz, has said that the doctors and nurses involved should not return to their profession.

"It is in the interests of all doctors that those who are responsible for this are punished," the minister added.

Investigators are also probing the possibility that the medical staff may have also have deceived the pharmaceutical companies that commissioned the trials.

The suspects said that the all those involved knew that the trial involved an anti-H5N1 drug and willingly participated.

The news of the investigation will come as another blow to the reputation of Poland's beleaguered and poverty-stricken national health service. In 2002, a number of ambulance medics were found guilty of killing their patients for commissions from funeral companies.