Sussex Ramblers is threatening court action over what they argue is the illegal blocking of a public right of way.
The route between Brighton and Peacehaven has been blocked by a fence erected by Southern Water, with parts of the path being dug up.
However, the walking charity says it has no right to stop people who want to go walking
Local footpath secretary Peter Seed said people had at first believed Southern Water were allowed to block the path.
"But then our research by David Brookshaw of the South Downs Society revealed that they had no such permission," he added.
East Sussex County Ciouncil has been unsuccessfully negotiating with Southern Water over the issue, with the Ramblers saying the time for talking is nearing its close - and court action should follow.
In October, Ramblers Cymru and the open Spaces Society were finally able to toast success in a campaign to remove an illegal fence in the Brecon Beacons that had remained in place since 2006.
The barrier had been due to come down that year, having been put in place during the 2001 foot-and-mouth outbreak.