Laurent Bonomo, 23, was stabbed almost 200 times with up to 100 injuries inflicted after he was already dead, sources said.

His close friend, Gabriel Ferez, 23, suffered around 50 injuries during a long ordeal at his friend's bedsit in south-east London.

Sources said both men were unable to move after being bound and gagged some time before 10pm on Sunday. Investigators believe an accelerant was poured on or near their bodies, possibly in a bid to destroy evidence.

The two young men are believed to have been playing a computer game together when they were attacked by one or more men in Sterling Gardens, New Cross.

Detectives believe a burglary at the rented flat six days earlier could be linked to the double murder. They said there was no evidence of forced entry and a set of keys may have been used, enabling an attacker to surprise his victims.

Detectives said Mr Bonomo, of Velaux, and Mr Ferez, of Prouzel, were "entirely innocent", with no criminal background in France or the UK. The bio-engineering students were due to return home later this month after a three-month placement at Imperial College London.

Detective Chief Inspector Mick Duthie told a press conference at New Scotland Yard: "I have never seen injuries inflicted to bodies like this before. The extent of the injuries are horrific. Everyone working on this case, including myself, has been deeply shocked by what we have seen."

"I have never seen injuries like this throughout my career. We are here today because I don't know why these boys were killed or who killed them. I do believe, however, that those responsible must have been blood-stained when they left."

Mr Duthie said police are hunting a white man seen running from the flats shortly after the explosion. But they have not ruled out that others may have been involved.