The train route between Hastings and Ashford could be at risk of closing in the future without High Speed 1, warns rail action group Marshlink.
The line narrowly avoided closure during the Beeching cutbacks.
Marshlink chairman Stuart Harland fears it could be facing the same fate when the current diesel units reach the end of their life.
He said: “Not wishing to be alarmist or sensationalist, there will be no more diesel units to replace them and there may still be no business case for electrification. Our rail system at the moment has several constraints. It is, substantially, a single-track and diesel-operated line surrounded by electric trains and track.
“The diesel trains are only 10 years old but are already suffering from some mechanical issues. With a service depot in south London which is only accessible via Hastings. So any train failures directly affect the service, there being no supporting units available from surrounding services. Meanwhile, passenger demand is growing so a current train operating at two cars per hour results in increasingly overcrowded trains. It was noticed when the Hastings line was closed last winter that commuters from Hastings and Bexhill were using the Marshlink to get to the HS1 service from Ashford and are probably continuing to do so.
“Rye and Hastings are tourist-dependent towns and improved transport links can only improve business and regeneration. A more frequent service would be more attractive for Eurostar passengers, so perhaps Ashford’s position as an international station could be restored.”