Allegations that the Princess of Wales was murdered are unfounded, the former Metropolitan Police commissioner Lord Stevens said today.
Prince William and Prince Harry want the "conclusive findings" of the investigation into their mother's death to bring an end to speculation surrounding the crash.
Clarence House said today the brothers were grateful for the thoroughness of the high-profile inquiry into the tragic accident that killed Diana, Princess of Wales.
Lord Stevens, unveiling the results of his exhaustive three-year high-profile inquiry into Diana's car crash in Paris, he said he had found nothing to justify further inquiries.
Lord Stevens also dismissed claims that Diana was pregnant when she died and that she was planning to marry Dodi.
Detectives were tasked with examining the persistent conspiracy theories surrounding the accident, including allegations that the Princess and her lover Dodi Fayed were murdered.
Diana, 36, and 42-year-old Dodi were killed when their Mercedes crashed in the Pont de l'Alma tunnel on August 31 1997.
A statement from Clarence House said: "Prince William and Prince Harry have received a copy of the report from Lord Stevens personally.
"They are extremely grateful to Lord Stevens and his team for the thoroughness and professionalism they have shown during their investigation, and trust that these conclusive findings will end the speculation surrounding the death of their mother Diana, Princess of Wales."
The Spencer family said today they agreed with the conclusions of the inquiry entirely.
Diana's siblings Earl Spencer, Lady Sarah McCorquodale and Lady Jane Fellowes issued a joint statement confirming their support for Lord Stevens' findings.
"We have been kept informed over the course of the inquiry by Lord Stevens and his officers," they said.
"We have been briefed on the conclusions of the inquiry and agree with them entirely, and look forward to reading the full report in detail."
Lord Stevens told a packed news conference in London: "There was no conspiracy to murder any of the occupants of that car. This was a tragic accident."
He revealed there was no evidence of any link between the Duke of Edinburgh and the Security and Intelligence Service (MI6), a reference to claims by Dodi's father Mohamed al Fayed.
Lord Stevens said Diana "was not engaged and was not about to get engaged".
Prince William told him that his mother had not given him the slightest indication of such plans for the future.
The investigation was the largest and most comprehensive survey of the crash scene yet and Lord Stevens said he satisfied no information had been held back.
He confirmed that there had been correspondence with the Duke of Edinburgh as part of the inquiry and that two new eyewitnesses had also been uncovered.
Lord Stevens said the primary purpose of his investigation had been to decide whether there was "credible evidence to support an allegation of conspiracy to murder" in relation to Diana's death.
He said that at the heart of Mr al Fayed's allegations was his belief that the crash was not an accident but was a murder resulting from a conspiracy by the establishment and in particular by the Duke "because of the relationship between Mr Dodi al Fayed and the Princess of Wales".
Lord Stevens said he had carried out "every reasonable line of inquiry" in order to evaluate whether there was "any evidence to support these extremely serious allegations".
Mr al Fayed and his legal team had also made allegations about the French investigation, claiming it was carried out in such a way as to prevent a proper examination of the accident, Lord Stevens said.
He said he had "personally examined" MI5 and MI6 records over a long period of time to try to establish the truth behind the allegations.
The inquiry team had also been in contact with the US intelligence services, who had given their assurance that they had no relevant information that would alter the findings.
Lord Stevens added: "We are confident that the allegations made are unfounded."
He said Diana and Dodi's driver Henri Paul had been drinking on the night of the crash, driving at excessive speed and had an alcohol level of around 1.74 grams per litre at the time of the crash - about twice the British drink drive limit.
In conclusion, he said: "I have no doubt that speculation as to what happened that night will continue and that there are some matters, as in many other investigations, about which we may never find a definitive answer.
"However, I do not believe that any evidence currently exists that can substantiate the allegation of conspiracy to murder that has been made.
"Various legal cases are currently being pursued by Mr al Fayed through the French courts.
"They are unlikely, in my opinion, to have any bearing on my conclusion that there was no conspiracy or cover up."
Lord Stevens added: "Three people tragically lost their lives in the accident and one was seriously injured.
"Many more have suffered from the intense scrutiny, speculation and misinformed judgments in the years that have followed.
"I very much hope that all the work we have done and the publication of this report will help to bring some closure to all who continue to mourn the deaths of Diana, Princess of Wales, Dodi al Fayed and Henri Paul."
Lord Stevens said he had no comment to make about Mr al Fayed's claims about his investigation, adding: "He's a genuinely grieving parent."
But he added that others were also grieving and "you have to draw a line under it and move on".
The inquiry report said DNA testing had confirmed that the blood samples which established that Mr Paul had been drinking were genuine.
Lord Stevens said that had Diana, Dodi and Henri Paul been wearing seatbelts, they might not have died.
None of the occupants - even bodyguard Trevor Rees-Jones - was wearing a seatbelt, reporters were told.
It was unlikely that the white Fiat Uno with which Diana's Mercedes collided in the Alma underpass in Paris would ever be traced, he added.
Lord Stevens was asked about the role of the paparazzi who were pursuing Diana and Dodi at high speeds through Paris when they crashed.
Princes William and Harry were left distressed and angry after learning in full from the report of the photographers' behaviour.
French paparazzi took pictures of the Princess as she lay fatally wounded in the wrecked Mercedes at the scene as emergency workers battled to save her.
Lord Stevens said: "A crash of this nature is similar to a major crash of an airliner. It is a long chain of events.
"Take any link out of that chain and this would not have happened."
But asked who was to blame for the crash, he said: "I lay no blame at anyone's door."