In the €2bn (£1.74bn) deal, confirmed on Wednesday, Eurostar has ordered 30 “Celestia” trains, with the option for 20 more.
Each train would be 200m long. If two were run together, as happens currently, the resulting 400m service would have about 1,080 seats.
These would be the first double-decker high-speed trains to run through the Channel Tunnel.
The only other double-decker to ever appear on Britain’s railways was an experiment that began in 1949.
Eurostar’s chief executive Gwendoline Cazenave said she was “particularly proud to bring double-decker trains to the UK for the very first time”, adding they would bring “exceptional comfort”.
Eurostar carried 19.5 million passengers last year. Ms Cazenove said the company aimed to grow this number to 30 million.
The firm said a fleet of new trains, some of which will replace older ones, will lead to a 30% increase in trains that service London.
The double-decker train deal comes after Eurostar announced an expansion plan in June that includes new routes to Geneva and Frankfurt from London.
