The companies, which include contractor Interior Services Group, architect PRP and M&E engineer Troup Bywaters + Anders, are renovating part of the hospital for the Isabel Medical Charity.
The team has been put together by Scott Lawrie, an associate director at architect PRP, whose son's life was saved by staff at St Mary's after he contracted meningitis.
The building will provide a headquarters for the charity, which was set up to help junior doctors avoid the misdiagnosis of rare childhood diseases.
Five subcontractors have also agreed to assist in the renovation work free of charge. MSK, a labour agency, has agreed to provide general building workers; Loughton Carpets is donating carpets; Epoole, a decorating company, is undertaking the decoration work; Lenval, an asbestos removal specialist, and Bricks and Water, which provides mechanical and building services, have also agreed to help.
When the work came up, I couldn’t think of anyone better to approach
Joseph Britto, St Mary’s Hospital
Lawrie's four-year-old son Fergus was successfully treated for the killer disease meningitis in the intensive care ward at St Mary's a year ago.
Joseph Britto, a consultant and paediatrician in the intensive care unit, was one of the doctors who treated Fergus.
He said: "When Fergus was admitted to intensive care, every system in his body was failing. But he came through, and Scott and his wife Pamela said that they would like to support the intensive care unit in some way.
"When the building work came up, I couldn't think of anyone better to approach. Scott is doing this without any cost to the charity. He is not only giving something back to the intensive care unit, but to the Isabel Medical Charity and children that may be at risk from misdiagnosis."
Lawrie knew that Interior was already doing some work for St Mary's in Paddington – a fit-out of one of their laboratories. He said: "I knew that Interior had a relationship with St Mary's and was working in the area. It had management support set up nearby, and that made it easier to get it on board."
The Isabel Medical Charity was set up by Britto and former City broker Jason Maud, whose daughter Isabel fell ill with a rare disease. She almost died because she was misdiagnosed.