The mystery surrounding the circumstances of Princess Diana’s death deepened today as details emerged of an unidentified car which had been deliberately blocking the path of her speeding Mercedes moments before the fatal crash in the Pont de l’Alma tunnel.

The inquest into her death, and that of her Egyptian boyfriend Dodi al-Fayed, heard that a “blocking vehicle” had been trying to slow down the Mercedes as it entered the underpass, apparently so that pursuing paparazzi could take photographs.

The car, along with several motorcyclists, formed a “compact group” around the Mercedes, said Olivier Partouche, then a chauffeur who witnessed the incident while waiting for a client on the roadside.

“I was just of the impression of a kind of a group, with in the front of the group the car and the motorcycles just behind at the same speed,” he told the High Court jury by videolink from Paris.

He described the vehicle in question as a dark-coloured Ford Mondeo-style mid-sized saloon, but said he could not determine the make.

In a statement to French police made shortly after the crash and read out at the inquest, Partouche claimed the Mercedes had been travelling at around 150km/h (93mph).

Today he offered a revised estimate, saying he now believed it was slower than this but still “very, very fast.”

“In front of the Mercedes was a car, of which I could not tell you the make,” he said.

“It was dark in colour and clearly this car was trying to make the Mercedes slow down.

“The Mercedes was black. I think it was an S Class 500.

“The object of the manoeuvre was to make it possible for the paparazzi to take photographs.”

In a later statement to a French examining magistrate, also presented at the inquest, Partouche made clear he did not believe the manoeuvre had caused the Mercedes to crash. The Princess of Wales’ car had ultimately pulled out past it, he said.

“The vehicle in front of the Mercedes did not perform any dangerous manoeuvres on the road to prevent it (the Mercedes) getting past.”

Diana, Dodi and their driver Henri Paul, were killed in the Paris crash on August 31, 1997. Dodi’s father, Harrods owner Mohammed al-Fayed, has always maintained that they were murdered at the instigation of the Royal Family in order to prevent the Princess marrying a Muslim and giving birth to the child he claims she was carrying.

Though the inquest has seen CCTV footage of Dodi buying an engagement ring hours before the fatal crash, British and French police have long ruled out any conspiracy, attributing the crash to the fact that Henri Paul was drunk and speeding.

On Monday, witness Alma Samer told the inquest she had seen a car parked at the entrance to the tunnel as she drove into it in the aftermath of the crash. As she passed the scene of the accident, two men broke away from the group standing around the Mercedes and walked back towards the stationary vehicle, which she described as large and dark in colour, possibly a Mercedes or Audi. They then drove away, she said.

However Francois Levistre, who testified in front of the inquest on 15 October, claimed he saw two men on a motorcycle ahead of the princess’ car, which crashed after “a major flash of light.”

The passenger on the motorcycle then looked into the crumpled Mercedes and gave a two-hand gesture to indicate “job done,” before it sped off, he alleged.

Other French witnesses have reported seeing two cars enter the tunnel side by side, but did not recall seeing a bright light.

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