Experts have warned the initiative risks breaching competition rules.
The TV chef has met other restaurant bosses urging them to follow his lead in slapping a 10p tax on all added sugar drinks.
So far two other firms, Leon and Abokada, have rallied to the call. But lawyers say it could be seen by the Competition and Markets Authority as price fixing.
Dominic Watkins, from law firm DWF, said: “Discussions about selling price between businesses at the same level of the supply chain carry a significant risk of being anti-competitive.
“A breach of competition law can result in fines and imprisonment.”