The imprisoned son-in-law of a murdered Monaco billionaire has gone on trial today with nine others suspected in the brazen daylight killing of the heiress.

Helene Pastor, 77, was shot dead in Nice on the French Riveria in 2014 in a case prosecutors say bears all the hallmarks of a crime thriller.

On May 6 that year, an unknown gunman with a sawed-off shotgun fired into her car as she left a hospital after visiting her son.

Wojciech Janowski, 64, (left) withdrew his confession to the murder of Helene Pastor, 77, (right) saying he did not understand 'all the nuances' of the French language

Sylvia Janowski, daughter of Helene Pastor, leaves the court room in Aix-en-Provence today

The chauffeur died a few days later and Pastor fell into a coma.

She awoke briefly and was able to tell investigators: 'I'm afraid, I want to see you again because I have more to tell.'

Before she could, she died of her injuries on May 21.

Police quickly identified two men suspected of carrying out what they now believe was a contract killing orchestrated by Wojciech Janowski, 69, Pastor's son-in-law.

After initially admitting to ordering the killing to get his hands on Pastor's estimated £10bn fortune, Janowski later retracted his confession, saying he had misunderstood investigators' questioning.

The suspected killer, Samine Said Ahmed, 28, is also on trial and has denied the charges.

Al Hair Hamadi, 35, is accused of acting as a lookout.

Janowski and Sylvia Pastor (right) have been in a relationship for 28 years and have a teenage daughter

A police officer investigates a car at the site where Helene Pastor and her driver were shot dead

Wojciech Janowski, hides his face as he leaves a police station in Nice, France, after questioning in 2014

Prosecutors say the two men, originally from the African Comoros islands and living in northern Marseille, left behind a multitude of clues.

They were caught on CCTV in Marseille buying mobile phones and boarding a train for Nice, where their DNA traces were found on a bottle left in a hotel room.

They are also seen waiting for Pastor opposite the hospital, having arrived by taxi after unsuccessfully trying to purchase a scooter for their getaway.

'It's the first time we've seen killers use a taxi to reach the scene of a crime - they did everything you're not supposed to do,' a source close the inquiry said.

Pastor had inherited a huge real estate and construction business set up by her Italian grandfather Jean-Baptiste Pastor, a stone mason who moved to Monaco in 1880.

Phone records also implicated several suspected accomplices who will also be on trial in Aix-en-Provence, just outside Marseille.

The hearing is expected to run until October 19.

Forensic experts investigate the car in which Helene Pastor was fatally injured on May 6, 2014

Mrs Pastor has been visiting her son in a hospital in Nice before the shooting

Witnesses said the gunman used a large pump-action shotgun in the hit, in which both Helene Pastor and her chauffeur were killed

Janowski has been married to Pastor's daughter Sylvia, 57, for 28 years, and was a businessman and Poland's honorary consul to Monaco before being stripped of the title following the charge.

A graduate of the University of Cambridge, he headed a nanotechnology firm and an oil business, and was said to be involved in numerous charities in the principality.

But prosecutors say his business ventures were not as successful as he claimed, and he was hoping the inheritance would help him recover even though he already received part of the €500,000 Pastor gave her daughter every month.

The fatal shooting of a billionaire heiress on the idyllic French Riveria led prosecutors to say the case bears all the hallmarks of a crime thriller

A month after the murders, Janowski's personal trainer, 49-year-old Pascal Dauriac, admitted to organising the hit with his brother-in-law.

'Janowski tricked me... He said his mother-in-law was a monster,' Dauriac said.

He also claimed Janowski himself had scouted out the hospital and wanted to have Pastor's son killed too.

Janowski allegedly spent €250,000 to hire contract killers.