A man left brain-dead after a drug trial in the French city of Rennes has died, according to local media reports.

He was one of six people hospitalised following the trial.

The other five remain in a stable condition, with four showing "neurological problems," the hospital said in statement.

The trial involved a drug manufactured by Portuguese company Bial.

90 people were given the experimental drug in various doses, and there is no known antidote.

"A serious accident took place during a therapeutic trial near Rennes," French Health Minister Marisol Touraine said.

Testing company Biotrial said in a statement that "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," the statement adds.

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

Ms Touraine expressed her "deep determination to get to the bottom...of this tragic accident".

The first volunteers to fall ill were taken into hospital earlier this week, French media said.

A formal investigation has been opened in Paris.