French prosecutors demanded long prison sentences on Monday for an Italian restaurant manager and a hard-up British ex-soldier who are accused of kidnapping an ageing hotel heiress in the city of Nice.

The men have been on trial along with 11 other alleged accomplices since January 4 over the plot against Jacqueline Veyrac, the owner of the five-star Grand Hotel in Cannes and the ritzy seafront La Reserve restaurant in Nice.

Chief prosecutor Annie Brunet-Fuster suggested a 30-year sentence for Italian restaurant owner Giuseppe Serena, who had long denied responsiblity but appeared to make a confession in court.

"For me, he is the instigator, whatever he says," she told the court.

The prosecutor also called for a 25-year sentence for former British special forces soldier Philip Dutton, 52, who has admitted involvement in the kidnapping.

Veyrac, 80, was snatched on the street on October 24, 2016, and bundled into the back of a van.

She was held bound and gagged for two days in the vehicle, which was parked on a quiet street, before being spotted by a passerby and released.

Serena, a former manager of Veyrac's La Reserve restaurant, allegedly bore a fierce grudge against the restaurant owner after she terminated his contract in 2009.

Serena is accused of ordering her abduction with a view to obtaining a ransom with which he hoped to open a new restaurant.

The chief prosecutor also demanded jail terms of 18-20 years for three men accused of being the main accomplices to the kidnapping.

A private detective who admitted to attaching a GPS device to Veyrac's vehicle, should be given an eight-year sentence, the prosecutor suggested.

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