The son-in-law of a Monaco heiress was due to go on trial on Monday accused of plotting her murder to get his hands on her €12bn fortune in a case described as being straight out of a crime thriller.

Hélène Pastor was killed in an ambush on her chauffeur-driven car in broad daylight in May 2014. She was leaving a Nice hospital after visiting her son when a masked gunman fired shots through the vehicle’s windscreen, fatally injuring her and her driver.

While lying in intensive care in hospital, Pastor, 77, woke from a coma to announce to detectives: “I’m afraid, I want to see you again because I have more to tell,” but she died of her injuries before she could do so.

Pastor’s son-in-law, Wojciech Janowski, 69, a businessman and honorary Polish consul to Monaco, was arrested and accused of organising a contract killing. Janowski confessed but later retracted the confession, telling police he had misunderstood “the meanings of the terms used by police”.

At the trial opening, Janowski, wearing a dark suit and tie, told the court he would answer all questions. Formally accused of involvement in Pastor’s killing, Janowski said: “I am innocent. I’ve committed no crime. That’s all. Thank you.”

Pastor was hit several times by shots from a sawn-off shotgun as she left the hospital where her son Gildo, then 47, was recovering from a stroke. Her chauffeur of 15 years, Egyptian-born Mohamed Darwish, 64, died a few days later of his injuries.

The alleged gunmen were quickly identified and police began to suspect a murder plot. 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.

Marseille’s public prosecutor, Brice Robin, told journalists that although Pastor did not like Janowski, he had obtained €8.4m from her via her daughter Sylvie in the year before the killing. Robin said the money was not in Janowski’s bank account and he had no idea where it had gone. He said the couple, who had lived together for 28 years, received generous allowances of up to €500,000 a month and lived in luxury apartments owned by Pastor.

Sylvie was arrested by police after the killing but was later released. She told detectives she had planned to visit her brother in hospital with her mother but that Janowski had expressly forbidden her to go. Among nine other defendants on trial is the couple’s personal trainer, Pascal Dauriac, who told police the businessman had “manipulated” him to organise the crime

Pastor owned a property empire in Monaco worth an estimated €20bn, established by her grandfather, an immigrant Italian stonemason, and she had links with the royal family.

Before Pastor’s killing, Janowski, a former casino croupier who became the Polish honorary consul to Monaco in 2007, was known principally for his charity work, particularly with the organisation Monaco Against Autism, which he co-founded. The charity has Princess Charlene of Monaco as its honorary president.

Janowski was given the National Order of Merit of the French Republic by Nicolas Sarkozy in 2010 for his charity work. In the past he ran a chain of hotels and casinos in the principality.

The court was told he had a false diploma from Cambridge and his companies made little or no money.

The trial continues tomorrow and is expected to last five weeks.