Wojciech Janowski, son-in-law of Monaco heiress Helene Pastor, hidden by the arm of a policeman, while leaving the courthouse in Marseille in June 2014. File picture: Patrice Masante/Reuters

Paris - The son-in-law of murdered Monaco property billionaire Helene Pastor will Monday go on trial in France for her 2014 contract killing, along with nine other defendants.

In one of France's most sensational crimes in decades, Pastor was shot along with her chauffeur in May 2014 after leaving a hospital in Nice where her son Gildo Pallanca-Pastor was being treated after suffering a stroke.

Pastor, a close friend of the Monaco royal family, died a fortnight later. Her chauffeur succumbed to his injuries four days after the shooting.

Investigators believe her son-in-law, Wojciech Janowski, who was Poland's honorary consul to Monaco at the time, orchestrated her killing. He was arrested a few weeks after Pastor's death and made a confession, which he however retracted shortly afterwards.

In an interview with the Nice-Matin newspaper earlier this year, Janowski said he had made the false confession to protect his wife from being detained and because of a language misunderstanding.

Thomas Giaccardi, a lawyer for Pallanca-Pastor, told dpa this was ridiculous as the son-in-law could understand French perfectly.

Pallanca-Pastor told the Le Parisien newspaper he planned to attend the entire trial, which is scheduled to run until October 19, and expected the accused to be convicted.

Among the others on trial are the son-in-law's fitness coach, who organized the murder for him, as well as the suspected shooter and an accomplice who was believed to have been with him at the site of the attack.

The shooter and his accomplice were identified through security camera footage, mobile phone data and DNA traces left on a bottle of shower gel in a hotel room.