Regional authorities in Saint-Nazaire harbour, France said up to 50 visitors were walking onto the liner across a gangway when the structure collapsed under them
The Queen Mary 2 is undergoing final stages of construction at the Chantiers de l'Atlantique shipyard, owned by Alstom, a French heavy engineering firm.
Alstom officials said the visitors were part of a larger group that is permitted on the liner every Saturday.
"For each shipyard worker, the construction of a cruise liner is a source of pride to be shared with friends and family," said Philippe Bouquet-Nadeau, the head of human resources for Alstom Marine. "That is why we always authorise these visits. It is a very old tradition."
"Never have we seen such a tragedy," Saint-Nazaire mayor Joel Batteux told reporters, choking back tears. "How could these people die at the foot of our town's proudest symbol?"
About 50 rescue vehicles and one helicopter descended on the scene. An emergency hospital was set up at the base of the massive ship and psychologists attended to family members of the victims.
Outside the hospital in central Saint-Nazaire, a modest port town where one in 10 people works for the shipyard, dozens of anguished faces hovered waiting for news on their relatives.
"My daughter is in a state of shock," said Muriel Leduc, the wife of a shipyard worker, as she left the emergency room. "She was standing on the dock, awaiting her turn when the platform collapsed. She heard the cries of the injured."
The ship, which is being built at a cost of around $800 million, has been conducting sea trials in recent weeks.
"How could these people die at the foot of our town's proudest symbol?"
The largest passenger ship ever built and flagship of the Cunard Line, Queen Mary II stretches the length of four football fields and stands as high as a 23-storey building.
The original Queen Mary entered service on Cunard's prestigious Atlantic route in 1936, becoming one of the best known ships of the golden age of liners.
The Queen Mary II completed its final sea trials off the Brittany coast last week. Its maiden voyage from Southampton, England, to Fort Lauderdale, Florida, is set for 12 January.
The ship has an onboard planetarium and art gallery and will carry about 3000 passengers on its 15-day maiden voyage.