Eurostar has been calling for state bailouts and has stripped its services to the bare minimum, with just one return journey per day.
Dozens of members of the European Parliament are urging the French and British governments, as well as the European Commission, to step in to save Eurostar.
The COVID-19 pandemic and travel restrictions have battered the finances of the cross-Channel train service, causing passenger numbers to collapse by 95 per cent since last March.
Eurostar has been calling for state bailouts and has nowadays stripped its services to the bare minimum, with just one return journey per day.
The firm, which is 55 per cent owned by the French state rail company SNCF, is currently in talks with the French and British governments for financial support.
"The future of rail travel across the Channel is at stake if no solution is found," reads the letter, which more than 50 MEPs have signed.
"This would not only negatively impact economies on both sides of the Channel and lead to significant job losses, it would also seriously undermine efforts to make transport more sustainable in Europe."
It argues that governments pledging a more environmentally friendly future should not ignore the difficulties facing the international train service while bailing out airline companies.
Barry Andrews, an Irish MEP with the Renew Europe group who signed the letter, told Euronews that the European Commission had a "critical role to play" in rescuing Eurostar, given its commitment to building a greener and more digital economy.
"This is a crucial piece of rail connectivity infrastructure that cannot be allowed to collapse," Andrews said in an interview.
"It's also a symbolic connection between the UK and the EU," he said. "It would be tragic and would compound the injury of Brexit if we lost the Eurostar and the Channel Tunnel connection because of a lack of investment."
Watch the interview with Andrews in the video player at the top of the page.