Forensic scientists working for Lord Stevens, the former police commissioner leading the inquiry, are studying samples taken from the body of Henri Paul in an attempt to explain the unusually high levels of carbon monoxide found in his blood.

The news came after a week of fresh allegations surrounding the inquiry . . . the latest in a series of claims, counter-claims and conspiracy theories which have swirled around the crash in a Paris underpass for nearly a decade.

Tests carried out during the official French investigation found that Paul's blood alcohol levels suggested he was drunk at the time of the accident.

But they also revealed that his blood contained over 20% carbon monoxide (CO) . . . a level which some medical experts say is consistent with smoke inhalation or suicide.

Investigators for the Stevens team are now re-examining the sample to find other possible explanations, confirmed a member of the legal team representing Mohammed al Fayed, whose son Dodi also died in the crash.

"The police have reached the conclusion that the French explanation does not hold water, " he said.

The French inquiry concluded that the high CO levels could have been caused by a ruptured airbag. But Fayed's team say airbags do not usually contain CO, and that Henri Paul was killed on impact, so would have been unable to inhale the gas.

The British investigators are examining three possible scenarios, the source said. "First, that there is a rational explanation for how such high levels of carbon monoxide got into his blood; second, that the results were mistaken, which casts doubt on the other results of the blood tests; or third, that it might not be his blood, " he said.

Fayed has long maintained that Henri Paul's blood samples were substituted with those of another body to cover up a British intelligence plan to murder Diana, who he believes was pregnant with Dodi's child.

Even before Princess Diana was buried in September 1997, the first whisperings of doubt emerged over official reports that she had been the victim of an ordinary car crash. An exhaustive French investigation concluded in 1999 that there was no evidence of a criminal conspiracy, but failed to scotch rumours of foul play.

The most recent round of claims include allegations that CIA sources told the Stevens inquiry that two diplomats at the British embassy in Paris were MI6 operatives . . . and were directly involved in the "murder" of the princess. Both men have reportedly denied any involvement, each saying he was not even in Paris at the time of the accident.

Another report alleged that Robert Fellowes, then the queen's private secretary, ordered a wireless operator to leave his post at the communications room in the British embassy in Paris half an hour before the accident. Lord Fellowes denies the allegation, saying he was in Norfolk at the time, with his wife . . . Diana's sister, Lady Jane Fellowes.

A third allegation is that a French legal official, Maud Morel-Coujard, has told the Stevens inquiry that "British authorities" ordered Diana's body to be embalmed before it was returned to England, which would have foiled any post-mortem pregnancy test.

Friends of Diana have denied she was pregnant, however, and British forensic sources say the French pathologists merely "touched up" the body to make its appearance less distressing for Diana's family. This mainly cosmetic treatment would have had no impact on a pregnancy test, they say.

Possibly the most incredible claim is that the RAF crew which flew Tony Blair from his Sedgefield constituency to meet Diana's body in London had been on standby for two days before her death. If the allegation were true, reported the Daily Mail, "it would mean that Diana's death was not only expected, it was planned".

Such scenarios may seem the stuff of fantasy, but last week Stevens made it clear that his team is taking every allegation seriously, however outlandish it may at first appear. Stevens, who previously investigated collusion between loyalist paramilitaries and British security forces in the North, said the inquiry has already proved "much more complex" and timeconsuming than he envisaged.

"We had to investigate every conspiracy theory and rumour to prove or disprove them. We're not just looking at an incident in a tunnel in Paris. We're looking at that, and beyond all that, " he said.

Advances in forensic technology have also added to the team's workload. The investigators plan to use virtual reality technology to recreate Diana's last journey from the back door of the Ritz Hotel to the moment the Mercedes she was travelling in crashed into the 13th pillar of the Alma tunnel.

The car itself is also being reexamined. Taken apart "bolt by bolt" in a laboratory in Camberley, England, the crash vehicle may yield important new evidence, including blood samples which may reveal if Diana was pregnant at the time.

Stevens regularly briefs Fayed on the progress of his investigations, and both men have spoken of their mutual respect.