Mohamed Al-Fayed could ‘burst into tears’ at the mention of his son Dodi after he died in the same car crash that killed Princess Diana, a documentary has revealed.

Speaking on the second episode of Investigating Diana: Death in Paris, Mr Al-Fayed’s former spokesman Laurie Mayer told how the billionaire was convinced his son and Diana were ‘assassinated’ and opened his own private investigation into the Paris car crash.

Recalling the first anniversary of the deaths of Diana and Dodi, Mr Mayer said people close to Al-Fayed were ‘incredibly nervous’ at how he would handle the milestone.

Laurie Mayer reveals Al-Fayed would cry at mention of his son

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Laurie Mayer, former spokesman for Mohamed Al-Fayed, said his old boss ‘could burst into tears’ at even a passing mention of his son Dodi

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The former Harrods owner (pictured with Princess Diana) opened his own private investigation into the 1997 crash which killed the Princess of Wales and Dodi Fayed

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Dodi Fayed was travelling in the car with Princess Diana when it crashed in the Pont d’Alma tunnel in Paris, which killed them both

He told the programme: ‘Everybody was treading on eggshells. He would quite unexpectedly burst into tears because somebody had made a passing reference to Dodi.

‘He’d be in the middle of a serious meeting and he was just sobbing. It was very, very painful to watch.’

The documentary showed footage from the day the former Harrods boss unveiled a statue in his Knightsbridge luxury department store in memory of Diana and Dodi.

Speaking to camera crews at the time, Al-Fayed said: ‘It’s a difficult day for me.’

Mr Mayer added the relationship between Dodi and Diana had ‘brought with it all the acceptance [Mohamed Al-Fayed] himself had always craved’.

PI Michel Kerbois: bright flashlight could blind driver and crash

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Michel Kerbois (pictured) was one of the investigators recruited by Mohamed Al-Fayed to examine the circumstances of the crash which killed Princess Diana and Dodi Fayed

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Martine Monteil, who was head of the Brigade Criminelle at the time of the accident, dismissed Mr Al-Fayed’s theory as a ‘conspiracy’

The episode explored the billionaire’s own investigation into the fatal 1997 car crash in which he enlisted private investigators to help find out what happened to Diana and Dodi.

Mr Mayer said his former boss believed his son had been the victim of an ‘assassination’ and worked to prove his own theory.

He said: ‘There was not a shadow of doubt in his mind that it was an assassination’.

However, investigators from the French authorities dismissed Al-Fayed’s theory, claiming it was a ‘conspiracy’.

Martine Monteil, who was head of the Brigade Criminelle at the time, said Al-Fayed ‘saw evil everywhere’ and that he was running a ‘counter-investigation’.

She said: ‘I only had respect for his pain. Not for his excesses.’

Among Al-Fayed’s group of investigators, which included former officers in the French Brigade Criminelle, was Michel Kerbois.

Insisting he was not a conspiracy theorist Mr Kerbois said: ‘I wasn’t working against them. I was working for the truth.’

Despite Mr Al-Fayed’s belief that his son had been murdered, Mr Kerbois insisted he ‘wanted to remain objective’ throughout his investigation.

Two years after the crash in 1999 the French authorities released their findings which concluded Princess Diana and Dodi Fayed had died in an accidental car crash.

Recalling the release of the findings, Mr Kerbois dismissed the report as ‘total rubbish’.

The second episode of the documentary series also interviewed Egyptian journalist Hala Sarhan, who developed a close working relationship with Mohamed Al-Fayed.

Ms Sarhan claimed Mr Al-Fayed had allowed her to see Dodi’s apartment, which had been kept exactly as it was when he was alive in what she described as a ‘shrine’.

She also claimed the billionaire had told her: ‘They didn’t like my son. We have curly hair. We have dark eyes. We are Muslims.’

However despite Mr Al-Fayed’s strong belief that his son and Princess Diana were murdered, French investigators continue to dismiss his claims.

Ms Monteil said: ‘The whole world has struggled to accept that The Princess of Wales died in a mundane accident.’