A botched drug trial in France may have left three people with "irreversible" brain damage, a doctor at the hospital has claimed.

Professor Gilles Edan, the chief neuroscientist at the hospital where the trial took place, said four men have "neurological problems", of whom three could have "irreversible" brain damage, another of the men has been declared brain-dead.

One of the men is being kept in hospital, but is in a less critical condition.

Six previously healthy medical volunteers are now in hospital after taking part in the botched drug test at a private clinic in western France, the Health Ministry said.

The drug trial, which was testing a new painkiller compound, involved 90 healthy volunteers who were given the experimental drug in varying doses, France's health minister Marisol Touraine told a news conference in Rennes.

Professor Edan said there is no known antidote to the experimental drug.

The prosecutor's office has opened an investigation into what Ms Touraine called "an accident of exceptional gravity ... without precedence" in France at the Biotrial lab in Rennes.

The six men in hospital, aged between 28 and 49, were healthy when the trial began on 7 January, Ms Touraine said, adding that one man now classified as brain-dead was admitted to Rennes Hospital on Sunday.

Biotrial, a research company for the reported drug manufacturers, Bial, have defended their drugs safety testing record following the incident.

A spokesperson for Biotrial said it was being conducted in “full compliance with international regulations” and its own procedures, which includes health monitoring.

The drug was based on a natural brain compound similar to the active ingredient in marijuana, Professor Eden confirmed.

Ms Touraine said the medication was not based on cannabis, as some media reports claimed. She urged calm, saying that no drug on the market was implicated in the failed trial. She said the drug was produced by the Portuguese pharmaceutical company Bial.

All the other volunteers are being contacted, Ms Touraine said.