A section of Lovers' Walks in Matlock Bath was fenced off following a report prompted by a rock fall.
It could force the cancellation of the Derbyshire town's illuminations which attract thousands of people.
Lovers' Walks are a series of paths on the River Derwent and over the cliffs and are believed to be the oldest surviving public pleasure grounds.
The closure includes the stretch from New Bridge in Derwent Gardens to Jubilee Bridge.
Experts found blocks of limestone ranging in size from 1cm to 2m wide were loose in the cliff.
A spokesman for Derbyshire Dales District Council said: "The report, which we commissioned after what we hoped was an isolated incident of a number of  loose rocks rolling down onto the footpath, is clear in its recommendation that public access to areas below the cliff face should be closed off immediately."
The riverside footpath is an emergency escape route during the illuminations and the closure could force the event's cancellation.
The spokesman added that repairs could be "extremely expensive" although more surveys were needed to see what work is required.
The original Lovers' Walks was created before 1742 and has been used as a public space ever since.
A 250-year-old public footpath and a children's play park have been closed over fears a cliff could collapse.