Six volunteers have fallen ill after participating in a clinical trial in France and one of them is brain-dead, officials said.

The French Health Ministry said the six were taken to hospital in the city of Rennes after taking a medication that was in the first phase of testing to study safe usage, tolerance and other measures on healthy volunteers.

The ministry did not name the medication being tested or the laboratory, saying only that it was a private operation that specialises in clinical tests.

Health minister Marisol Tourain travelled to the city after ordering an investigation into the organisation and the means and conditions under which it conducts clinical tests.

The testing company involved was named in reports as Biotrial.