Wojciech Janowski was Poland's honorary consul to Monaco at the time of the murder of the billionaire heiress Helene Pastor, 77, and her driver.
They were gunned down in her car by two hitmen in the southern French city of Nice on May 6, 2014.
At first Janowski admitted to orchestrating her death but he later retracted, saying his difficulties with French had made him misunderstand investigators' questions.
Yet after two weeks on trial with nine others suspected of playing a role in Pastor's death, Janowski's lawyer shocked the court Tuesday by saying his client "is guilty of ordering Helene Pastor's murder".
"These words which you wanted to hear from him come from my mouth. He tried to say these words, he wanted to say them but he couldn't," Eric Dupond-Moretti said as tears flowed down Janowski's face as he sat in the dock.
He argued that Janowski was trying to protect his partner Sylvia Ratkowski from her mother, who had made her life a misery, and that he had not ordered the two hitmen to kill her driver as well.
But that claim has been contested by Janowski's former personal trainer Pascal Dauriac, who is also on trial.
Dauriac says he was ordered to find the two killers and have them murder the driver and steal Pastor's purse to make it look like a robbery.
Prosecutors are seeking a life sentence for Janowski.
(Police investigate at the scene of the shooting. AFP)
'A big job'
The killing shocked the tiny principality of Monaco, where Pastor's family had built up a huge real estate portfolio of around 4,000 apartments, providing her with an estate worth some 12 billion euros ($14 billion).
Police quickly identified the two gunmen after they opened fire in broad daylight outside a Nice hospital.
Until Tuesday, Janowski had remained defiant under questioning, daring prosecutors to "show me the proof".
But prosecutors argued that the sharply dressed businessman was not as successful as he claimed, and was hoping to get his hands on the inheritance his partner stood to claim after her mother's death.
Dauriac, the former coach, said Janowski first broached the subject in 2012 and again in 2013, when Sylvia, a cancer survivor, became ill.
"This cannot go on, Sylvia's illness has deteriorated, we have to get rid of the old woman. Can you help me?" Dauriac quoted Janowski as asking him.
Dauriac claimed that Janowski plied him with gifts and free holidays to gain leverage over him and that he eventually gave in, asking his brother-in-law Abdelkader Belkhatir in nearby Marseille to help him find someone for a "big job" worth an alleged 140,000 euros ($165,000) in total.
He said his brother-in-law found two men for the job -- Al Hair Hamadi, who allegedly acted as lookout, and suspected gunman Samine Said Ahmed. Both men are among the ten on trial.