The son-in-law of the 77-year-old heiress of one of Monaco's richest families has admitted to involvement in her murder last month.

Wojciech Janowski, a 64-year-old who is also Poland's honorary consul in Monaco, "recognised his involvement" in the killing while in detention this week, Marseille prosecutor Brice Robin said.

Helene Pastor was shot multiple times as she was leaving a hospital in the French Riviera city of Nice.

Her only daughter, Sylvie, and her husband, Mr Janowski, were among 23 people detained in raids in Nice on Monday night.

Her driver, Mohammed Darwich, 64, also died from injuries sustained in the attack.

Mr Robin said police were also investigating "suspicious financial flows" in Mr Janowski's accounts.

He said two of the men being held, 31-year-old Alhair Hamadi and 24-year-old Samine Said Ahmed, had been identified as being "present at the location of the killing".

They were identified thanks to surveillance cameras, mobile telephone records and traces of DNA found in a Nice hotel where they stayed on 6 May, the day of the killing.

Significant amounts of cash were later found in the possession of the two men.

No further details were released about the possibility of any links between the two men, Mr Janowski and two other detained individuals who officials said may have acted as intermediaries.

Ms 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.

As the sleepy principality in the French Riviera slowly grew into a playground for the world's rich and famous, the family's fortune skyrocketed.

The real jackpot came in 1966 when Prince Rainier, whose fairytale wedding to Hollywood actress Grace Kelly helped catapult Monaco to international fame, gave permission to Helene Pastor's father Gildo to build high-rise buildings along the seafront.