Eurostar said on Wednesday it will run double-decker trains through the Channel Tunnel for the first time as it faces potential competition on its routes between London and mainland Europe.

A two-billion euro investment in up to 50 electric double-decker trains built by French group Alstom could help Eurostar reach 30 million annual passengers by the new decade, up around 50 percent from current traveller numbers.

It comes as other European operators are eyeing the cross-Channel route linking England with France, which could break Eurostar's three-decade monopoly on passenger services. Italy's Trenitalia, the UK's Virgin, Spain's Evolyn, and the Dutch startup Heuro have all indicated interest.

Eurostar chief executive Gwendoline Cazenave said the company's "milestone order" for Alstom trains "marks the concrete realisation of Eurostar's ambitious growth strategy – to reach 30 million passengers".

Eurostar on Wednesday confirmed an order for 30 trains, with an option for 20 more.

Cazenave said Alstom was chosen thanks to its ability to deliver new trains faster than competitors, the result of an existing contract with Eurostar's majority shareholder and French railway operator SNCF.

"We should receive all... [50] trains by the mid-2030s," she added.

Seat capacity is set to increase by 20 percent on each new train, to be known as Eurostar Celestia.

Alstom chief executive Henri Poupart-Lafarge said "this new-generation train, designed to meet the demands of international very high-speed traffic, embodies our vision of sustainable and competitive European mobility".

It is expected that each 200-metre set will have around 540 seats, while two can be joined to double the capacity.

The trains, to replace most of Eurostar's current fleet, will enter service in 2031, as the company looks to begin London-Geneva and London-Frankfurt routes.

This will add to the company's services between the British capital and France, Belgium, Germany and the Netherlands.

The London-Paris route, carrying eight million passengers each year, aims for two million more by 2030.

Virgin Group, the conglomerate founded by British entrepreneur Richard Branson, is hoping to launch a high-speed train service through the Channel Tunnel in 2029.

Britain's rail regulator earlier this year ruled that Eurostar's London maintenance depot, Temple Mills, could be made available to other operators. (AFP)