An armed man seized the gems from an exhibition of diamonds by the jeweller Leviev on Sunday morning.
Earlier the stolen gems had been given an estimated worth of 40m euros.
It is the largest in a series of high-profile robberies in Cannes.
Assistant prosecutor Philippe Vique, quoted by the Associated Press news agency, said the Dubai-based organiser of the diamond show had raised the value after doing a more detailed inventory check.
French police are looking for a lone suspect who broke in through French doors at the hotel wearing a mask and carrying a hand gun.
He helped himself to diamond encrusted watches, necklaces and earrings from an exhibition being held on the ground floor of the hotel and later fled on foot.
Hotel staff have complained about inadequate security at the exhibition, saying it was held in a wing that had direct street access.
Police also say they had not been alerted that the exhibition was taking place, which they say should have been standard procedure.
One line of inquiry is likely to focus on the Pink Panther network, a gang that has carried out gem raids in other countries. One of its members – a Bosnian – escaped from a Swiss prison on Thursday, with help from accomplices.
With the new estimate, the Cannes robbery would equal – or even surpass – one of the most valuable diamond heists in recent years at Antwerp Diamond Centre in Belgium in 2003.
Up to 100m euros worth of jewels vanished from the vaults of Antwerp’s diamond district believed to be carried out by an Italian gang.
The latest Cannes theft has also eclipsed a 2008 incident in France, when three men stole almost every piece on display at a jewellery exhibition in Paris with an estimated value of 85m euros.
The Carlton is located on the Croisette promenade, one of the most prestigious locations in the town.
Alfred Hitchcock used the hotel for his 1955 film To Catch A Thief, which portrayed a jewel thief operating in the French Riviera.
The hotel saw another robbery in August 1994, when gunmen burst into its jewellery store just as it was closing and made off with jewels then valued at £30m.