(Adds likely cause of death, paragraphs 5-6)

By Thierry Leveque

PARIS, July 2 (Reuters) - Chadian President Idriss Deby's son Brahim, touted as his possible successor, was found dead near his home outside Paris on Monday and police launched a murder investigation, French police and court officials said.

Brahim Deby's body was found early on Monday by a caretaker in the underground parking lot of a building where he lived in Courbevoie, to the west of Paris. He had a wound to the head.

A spokeswoman for the prosecutor's office in the western suburb of Nanterre said an autopsy had shown Deby had died "asphyxiated, probably by the white powder he was sprayed with."

The powder came from a fire extinguisher found beside Deby's body and the head wound he suffered was not the cause of his death, the autopsy concluded.

"He clearly died a violent death," a spokeswoman for the same office said. "We are going on the hypothesis of murder."

Brahim Deby was seen as his father's choice of successor but was widely disliked even by some of his own family who viewed him as unfit to govern, causing a split within the ruling clan.

The president sacked Brahim as his adviser in June 2006 after the then 27-year-old was arrested in a Paris discotheque for possessing an illegal firearm and drugs. He was given a six-month suspended sentence by a French court.

A coalition of rebel forces have been fighting a guerrilla war against Idriss Deby's forces in eastern Chad, saying he was fraudulently elected and demanding free, democratic elections to end his clan-based rule.

Makaila Nguebla, a Dakar-based spokesman for the rebel Union of Forces for Democracy and Development (UFDD), said Brahim Deby had been a key source of resentment who drove members of the president's administration to turn against him.

"He is at the root of all the frustration. He used to slap government ministers, senior Chadian officials were humiliated by Deby's son," Nguebla told Reuters.

"They had to leave the regime, go into the bush. They chose the military option instead of being humiliated inside Chad."

Nguebla said he believed Brahim's death would be a serious blow to his father's morale, particularly after one of his close nephews, the chief of Chad's armed forces, was killed in a clash with rebels last year.

"He was seen as a replacement for his father because (Idriss) Deby was often ill. If Deby had to be replaced it could only be him ... the morale of the family is completely destabilised," Nguebla said. (Additional reporting by Nick Tattersall in Dakar)

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