Eurostar has revealed plans to run double-decker trains through the Channel Tunnel for the first time.

The cross-Channel rail operator has confirmed it will order up to 50 trains from manufacturer Alstom, eventually increasing the size of its fleet by nearly a third.

The expansion plans would include investing heavily in a crucial London depot, it said.

But questions remain over whether the facility has enough space for both Eurostar and potential rival operators to use it.

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.

Eurostar also said it would invest €80m developing the Temple Mills depot in London, which is the only depot in the UK able to accomodate the larger trains used in continental Europe.

That depot, owned by the government’s London St Pancras Highspeed organisation, is currently exclusively used by Eurostar.

Eurostar is also the only company currently offering cross-Channel rail services between London and Paris, but others want to set up rival services.

Those companies include Spanish start-up Evolyn, Sir Richard Branson’s Virgin and a partnership between Gemini Trains and Uber.

The Office of Rail and Road is reviewing proposals from these firms to use Temple Mills, as well as Eurostar’s plans to increase services.

The regulator is expected to make a decision on access to the depot within weeks.