According to newspaper Corriere della Sera, the horseshoe-shaped concrete island will lie 1km offshore and will have moorings for 500 yachts and boast a luxury hotel, an aquarium, a conference centre and two swimming pools.
Tourists will travel to the £70 million 'island paradise' by ferry, although the architect behind the scheme, Sergio Peletti, has also drawn up plans for an underwater tunnel from the mainland.
However, the proposals have been attacked by local Green Party members, who believe the project will upset the 'marine panorama' and speed up the erosion of the beach which, it is claimed, is already at risk.
The leader of the Tuscan branch of the Green Party - known in Italy as the Verdi Party - Fabio Roggiolani, told the paper: 'Instead of flu, people have caught the virus of putting cement in the sea.'
Umberto Buratti, the head of the powerful Association of Bathing Establishments, was also unimpressed by the plans, which were officially unveiled yesterday.
He said: 'This seems like a Peter Pan island fantasy and I hope it stays like that forever.
An eruption of cement in the sea is dangerous. It doesn't take much to upset the delicate natural balance here.'