The Federal Service for the Oversight of Consumer Protection and Welfare, which is investigating the incident, was cited as saying that a total of 301 foreign and Russian construction workers have now been hospitalized in Moscow and its environs with food poisoning.

Earlier reports said the workers could have contracted Salmonella poisoning, a potentially lethal infection that affects the gastrointestinal system.

The service said food delivered for the workers was to blame, but added the source of the infection was still to be specified. The service also said the workers had their meals served in improper sanitary conditions, adding that, "Numerous violations of sanitary rules for cooking have been revealed during the investigation."

On Tuesday, when the first patients arrived in hospitals, the workers were served breakfasts and lunches – soft-boiled eggs, meat sandwiches, rice with lamb and carrots, onions and spices, and a salad – cooked by a private firm.