Twenty years ago this month people in Birmingham were waking to the dreadful news that Princess Diana had been killed in a car crash in Paris.

Her tragic death, in the early hours of August 31, 1997 led to a huge outpouring of grief among the British public who laid masses of floral tributes in her memory.

The tragedy happened in the Pont de l’Alma road tunnel in Paris, France.

Her lover, Dodi Fayed, and the driver of the Mercedes-Benz S280, Henri Paul, were pronounced dead at the scene; the bodyguard of Diana and Dodi, Trevor Rees-Jones, was the only survivor.

Although the media blamed the paparazzi following the car, and there were many conspiracy theories, an 18-month French judicial investigation found that the crash was caused by Paul, who lost control of the car at high speed while drunk.

Below, MirrorOnline look at some of the conspiracy theories still surrounding the crash...

SAS 'assassination plot'

One conspiracy theory that has not gone away two decades on from the fatal crash is that the SAS was involved in Diana and Dodi's deaths.

Scotland Yard launched a probe four years ago after an ex-soldier allegedly claimed that the princess was murdered by the special forces unit.

The former SAS sergeant, known as soldier N, was said to have told his wife that a special forces hit squad flashed a light to blind Diana’s driver and caused her death crash – using a technique developed to ­combat terrorists.

He went missing days before he was due to be quizzed by police, the Sunday People previously revealed, but later returned to the UK.

Soldier N's claims about Diana's death were first sent to police in July 2013 after the court-martial of SAS sniper Danny Nightingale.

Both men had been arrested in 2011 when officers raided the house they shared and found illegal guns and ammo.

Soldier N – who was mentioned in dispatches for his heroic actions defending a UN convoy after it came under fire during the Bosnian conflict in 1995 – was given two years’ detention for his part.

Nightingale’s conviction was quashed on appeal. But he was found guilty at a retrial and given a suspended two-year jail term.

In late 2013, the Met dismissed reports claiming SAS troops were behind Diana and Dodi's deaths, ­insisting there was “no credible evidence” of secret service involvement.

However, the claims are still believed by some today.

Mohamed al Fayed's claims

Dodi’s dad Mohamed al Fayed believes his son and Diana were murdered by an Establishment appalled at the idea of having a Muslim wed a royal.

A friend of the Harrods owner reportedly said several weeks ago: “Mohamed remains confident that information will emerge confirming his belief that Dodi and Diana were deliberately killed by the security services.

“He always says, ‘My son was slaughtered’, and he believes that was because the Establishment would not allow a Muslim to be married to the woman who would be the mother of the future king.”

Mr al Fayed has pledged to continue fighting for "the truth" about the couple's deaths "to be revealed". He has vowed: "I will never give up this fight."

The businessman previously expressed his belief that another car in the Paris tunnel blocked the princess's Mercedes as a Fiat and a bike followed.

However, a police expert told an inquest into Diana and Dodid's deaths that this conspiracy theory was a "non-starter". Speaking at the hearing, Scotland Yard investigator Anthony Read said that there was no evidence of another car.

It had been suggested that a photographer, now dead, working for M16, rode the bike and owned the Fiat. Witnesses saw a dog in the Fiat which, Mr Read said, would have had to be taken on this "murderous enterprise".

He said UK police would have searched the area for days.

A report by former Met Police commissioner Lord Stevens, published in 2006, rejected murder claims voiced by some, including Mr al Fayed.

Diana 'was pregnant'

Diana, Princess of Wales

In recent weeks, there have been continued claims that Diana was pregnant - or that Dodi was planning to propose to her - when she died.

The pregnancy claim was among those made by Mr al Fayed. It came to light during the inquest into the couple's deaths and was dismissed by a pathologist.

However, in 2007, a French investigative journalist alleged that Diana was "almost certainly" nine to 10 weeks pregnant when she was killed in the smash.

The journalist, Chris Laffaille, apparently claimed he had discovered evidence of the pregnancy from official hospital archives, the Daily Mail reported.

But responding to his allegations, a spokesman for the Paris Public Hospitals said the papers had been examined and were actually fake.

The proposal claim was also made by Mr al Fayed, who suggested his son and Diana had planned to announce their engagement.

However, there was no evidence that this was the case.

Unveiling the results of his two-year inquiry into the crash, Lord Stevens previously insisted Diana was not pregnant and had no plans to marry Dodi.

Speaking at a London press conference, he said: "There was no conspiracy - and no cover up. This was a tragic accident. I lay no blame at anyone's door.

"I have no doubt speculation will continue. There are some matters about which we'll never find a definitive answer.

"However, I do not believe any evidence exists that can substantiate the very serious allegation of conspiracy to murder."

At the time, Mr al Fayed branded the report "garbage."

'Tampered' car brakes

The tapes are set to be shown on British TV for the first time.

The hearing into Diana and Dodi's deaths heard that in 1995, the princess told several people that she believed the brakes on her car had been tampered with and that her life could be in danger.

However, the coroner said that despite her claims, there was "no evidence" to suggest she had then had her car checked for such tampering.

Mystery white Fiat Uno

A still image taken from the tapes.

Following the car crash, there were ­allegations a mystery white Fiat Uno could have collided with Diana and Dodi’s car.

Two weeks after the incident, French police confirmed the Mercedes had collided with another vehicle shortly before losing control.

White scratches on the Mercedes led investigators to conclude the car was a white Fiat Uno, made between 1983 and 1989.

However, Lord Stevens previously said paint tests showed it was not the nine-year-old car belonging to French photographer James Andanson, blamed by many for causing the crash.

Lord Stevens said: "We are completely satisfied the car was not his. We have spoken to his widow and on the night of the crash he was at home."

Three years later Andanson was found dead in a burned-out BMW.

It was decided he had committed suicide.

Mr al Fayed alleged he was working for the security services and was killed to keep him quiet. But Lord Stevens said there was no evidence he was an agent.

It later emerged that Le Van Thanh, 22, a taxi driver at the time of Diana's death, owned a white Fiat Uno identical to the car that struck the Mercedes.

Mr Thanh has always refused to shed any light on the crash and has turned down opportunities to talk to Scotland Yard during its investigations. In 2006, his dad said he had re-sprayed his white Uno red hours after the accident.

Mr Thanh reportedly refused to attend the inquest and was interviewed about the crash only by French police.

Earlier this year, MailOnline reported that Lord Stevens wanted to speak to the former cabbie because he believes his testimony could help disprove some common conspiracy theories about the princess's death.

Lord Stevens said: "What we have said to Mr Thanh is, 'We believe you were the driver of the Fiat. Talk us through what happened'.

"We don’t blame him for the accident. My Paget report said that a Fiat Uno was involved but was not the cause of the crash. We are still trying to interview him."

In a statement given to the inquest, he denied being the driver of the Fiat who fled the scene. He has allegedly refused to co-operate further.

A 'white flash'

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Soldier N was not the only person to allegedly make claims about a "light" in the tunnel following the tragedy on 31 August, 1997.

French witness Francois Levistre told the inquest into Diana and Dodi's deaths how he saw a "major white flash" in the underpass.

Mr Levistre, who was driving in front of the Mercedes at the time of the smash, described how the car was overtaken by a motorcycle.

He then claimed he saw a bright flash - like the light from a police radar - which was directed at Diana's vehicle, The Telegraph reported.

Speaking via video link from Paris, Mr Levistre said: "I realised there was this major white flash of the motorbike in front of the Mercedes, in front of the car.

"I was nearly at the exit of the tunnel and I realised that because I heard the noise of the motorbike within the tunnel. I just wondered what happened because the light was like you were caught by the police in a radar."

Ex-MI6 spy Richard Tomlinson also previously claimed a light was used to blind driver Henri in the tunnel. However, he later retracted his story.

'Other forces' behind the crash

Today, a retired lawyer who was one of the first witnesses to the crash said he believes “other forces” were behind the tragic accident.

Stanlee Culbreath maintained a dignified silence out of respect for then-young Princes William and Harry following their mum's death.

But now, almost 20 years on, he has shed new details on the incident, claiming Diana's chances of survival were greatly reduced by emergency service delays.

He also told The Mirror : “I always thought it was suspicious, that other forces played a hand, but now, 20 years on, I question more than ever whether it was a genuine accident. I just think it is dubious."

“If that’s the Princess, why did it take 20 minutes or so to get to her and, when she was finally released [from the car], why did they pass one hospital and take her to another?”

During the inquest, in 2007, it emerged it took an hour and six minutes from the time Diana was taken from the wrecked Mercedes until she reached the hospital.

The inquest heard the princess may have lived had French medics not “squandered” crucial minutes treating her at the scene.