The prospect of a merger between Eurostar and a rival operator on the Continent to create an integrated network spanning five countries has been given a “cautious welcome”.

The project, codenamed Green Speed, would see the Channel Tunnel passenger rail service combine with Thalys, the state-owned high-speed rail operator running between France, Belgium, the Netherlands and Germany.

The objective is to capitalise on environmental concerns over air travel by giving more passengers a sustainable alternative to flying.

Forecasts suggest it could boost the number of passengers across the combined network by two thirds over the next decade, from 18.5 million to 30 million.

A direct rail journey from London to Bordeaux would take about four-and-a-half hours.

The plan is being led by French state railway operator SNCF, which owns majority stakes in Eurostar and Thalys. SNCF presented proposals on Friday to its board, Belgian rail firm SNCB and Patina Rail, an Anglo-Canadian consortium that has a minority stake in Eurostar.

Eurostar said: “The management of SNCF, SNCB  and Patina Rail have presented the Green Speed project to their boards which proposes combining Eurostar, the cross-Channel high speed rail operator, and Thalys, the Franco-Belgian high speed rail operator, to create the foundations of a sustainable European high-speed rail travel company.

“When this project has been precisely defined, it will remain subject to the approval of the boards and consultation with the  representative bodies of employees as well as clearance of the European Commission regarding merger control.”

Guillaume Pepy, chairman of SNCF and Eurostar, told The Times: “The challenge of climate change and the demand for eco-responsible travel calls for an ambitious response.

“The creation of a combined European high-speed rail company would deliver a compelling alternative to road and air travel for our 18.5 million passengers and would herald a new era in the development of European high-speed rail services.”

Manuel Cortes, general secretary of the Transport Salaried Staffs’ Association, said: “Our union strongly supports high speed rail and therefore we give this plan a cautious welcome.

“It may well be that it points the way to an integrated, green, pan-European rail service which is in all our interests.

“However, I am now seeking a meeting with Eurostar and others in an effort to discover what the prospects are for jobs. There must be no jobs flight from Britain as a result of these plans, regardless of Brexit.

“The hope must be that the plans both improve rail services to and from the Continent as well creating jobs here.”