One person is brain dead and three others could have permanent brain damage after taking part in a drug test in France.

Six men aged between 28 and 49 were admitted to hospital, including the brain dead man, after taking the experimental drug.

A total of 90 people were given the drug in various doses, while about 30 others were given a placebo, said French Health Minister Marisol Touraine - who called it "an accident of exceptional gravity".

Three of those in hospital may have a "handicap that could be irreversible," said doctors in Rennes, where the men are being treated.

Chief neuroscientist at the city's hospital, Professor Gilles Edan, said a fifth man had neurological problems, while the remaining volunteer was in a less serious condition

There is no known way to reverse the drug's effects, said Professor Edan.

The painkiller compound - manufactured by the Portuguese lab Bial - was being tested by the research company Biotrial, which has offices in London.

The study was a Phase I clinical trial, where healthy volunteers take medication to evaluate its safety.

The patients had been in good health until taking the oral medication, said the French health minister.

The drug had been tested on chimps, but not on humans.

Ms Touraine expressed her "deep determination to get to the bottom ... of this tragic accident" and denied reports the drug was based on the compound found in cannabis.

"Undertaking Phase I studies is highly specialist work," said Daniel Hawcutt, a lecturer in clinical pharmacology at the University of Liverpool.

Medicines then go into larger Phase II and Phase III trials to assess their effectiveness and safety before they are finally approved for sale.

Biotrial said in a statement: "During (...) a study which was being conducted for a sponsor, serious adverse events related to the test drug have occurred in some subjects.

"The trial has been conducted in full compliance with the international regulations and Biotrial's procedures were followed at every stage throughout the trial, in particular the emergency procedures for the transfer of subjects to the hospital.

"We are in close and regular contact with the Health Authorities and Ministry in France.

"The priority at Biotrial remains the safety of our subjects.

"We are very grateful for the support we have been receiving from our clients and partners today."

Medical lawyer Philippe Courtois told French media: "This is the most serious case I have come across."

A formal investigation has been opened in Paris.

The emergency comes almost 10 years after six students were left seriously ill after a medical trial in London.

In that case, the six volunteers had a severe immune reaction which was triggered by the antibody drug, TGN1412.

They survived after intensive care treatment, but were told they faced a lifetime risk of cancer and arthritis.

One patient lost fingers and toes to gangrene following the trial at Northwick Park Hospital in London.

Another said his head swelled up so he resembled the lead character in the film, The Elephant Man.

The six were paid up to £2,000 to take part in that trial.