The first cases emerged on Tuesday and by the end of Wednesday had risen to 225, but the national food watchdog said the number of patients had now topped 300.
"A total of 301 acute infectious intestinal cases have been registered among construction workers from Turkey, Tajikistan, Uzbekistan, Moldova, and [Russia's] the Stavropol Territory," said the Federal Service for the Oversight of Consumer Protection and Welfare, which is investigating the incident.
All the patients are suffering similar symptoms: high temperature, nausea, vomiting and diarrhea. The patients are largely in a satisfactory condition, but some have contracted a severe form of a "food bacterial poisoning," the service said.
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.
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. The service also said the workers had their meals served in improper sanitary conditions.
"Numerous violations of sanitary rules for cooking have been revealed during the investigation," the service said.
The first patients arrived from a construction site on Tverskaya Street just a few steps away from Red Square and the Kremlin. Initial reports said poor-quality meat served as shashlik, or skewered meat cubes cooked over coals, sold by street vendors could have been to blame for the illnesses.
Workers from former Soviet republics are the main source of cheap workforce for numerous construction projects in booming Moscow, many of which are implemented by Turkish firms.
Poor working and living conditions of migrant workers, particularly from post-Soviet Central Asia who often stay in Russia illegally, have been continuously criticized by the Asian governments and rights groups.