A Total unit went on trial on Monday over a 2001 blast at a French chemical factory that killed 31 people and injured more than 2,000 in one of western Europe's biggest industrial disasters of recent years.

The Total unit, Grande Paroisse, which owned the AZF chemical fertiliser factory, and the plant's former director, Serge Biechelin, face charges including involuntary homicide and causing injury and destruction of property.

The explosion at the factory in an industrial zone on the outskirts of the southwestern city of Toulouse occurred less than two weeks after the Sept. 11, 2001 attacks on the United States and initially sparked suspicions of a terrorist bombing.

Judges subsequently found that there was no proof of a deliberate attack and that an accident was the most likely cause of the blast that damaged around 30,000 homes and hundreds of businesses in a radius of up to 6 kilometres (3.7 miles).

Investigators accuse the company of negligence in stocking volatile ammonium nitrate, charges which Total denies.

"We regret that the hypothesis of a chemical accident has been accepted and that not all possibilities have been thoroughly explored," the secretary general of Total, Jean-Jacques Guilbaud, said in a note included in court papers.

Total has already paid some 2 billion euros ($2.59 billion) in compensation but it says this does not constitute an acknowledgement of any criminal responsibility.

While Total has not advanced any firm theory of its own, other possibilities have been put forward, ranging from an electrical charge passed from a neighbouring plant to unexploded shells from World War One buried under the building.

The case has aroused deep feeling in Toulouse and bitter recriminations against Total, accused by associations representing families of the victims of obstructing the inquiry. Total denies this.

"Without the interventions of Total, we would have known the truth long ago," Gerard Ratier, whose son was killed in the explosion, told Reuters.

"Despite this trial, for my son, for so many other people grieving, I am afraid that this truth will be buried under a media circus that will continue to favour the defence."

Around 1,000 people have filed a civil suit alongside the criminal charges and the hearing, expected to last four months, will be held in a municipal hall capable of seating more than 1,000 people and, in an unusual step, recorded on video.