Holiday homeowners who have placed decking up against sea defences or dug steps into the shingle bank protecting part of the west Norfolk coastline have been told to remove them.
The Environment Agency (EA) has ordered their removal over concerns that it has been weakening the defences.
It comes a month before a report is expected which looks into whether the shingle bank protecting thousands of caravans and chalets between Hunstanton and Wolferton is being moved by the tides.
If a so-called tipping point has been reached, it could mean the start of a managed retreat, with properties being moved inland.
Now West Norfolk council's environment and community panel has been told letters had been issued to householders seeking removal of structures. If they don't, the work may be carried out by the Environment Agency, who would charge the property owner.
Warnings were first issued in October, when concern emerged over the future of the flood bank.
The EA said: "It is important that property owners refrain from digging into the ridge at all and that they approach the Environment Agency if they wish to undertake works within 16m of it.
"This will help preserve the integrity of the ridge and its ability to perform as a natural flood defence."
It comes as the EA prepares to carry out its annual beach recycling works, by collecting shingle which has washed south towards Snettisham Scalp and transporting it to bolster defences at Heacham.
Shoreline management plans commit the agency to "hold the line" of the current defences until 2025.
But the same documents warn that rising sea levels will eventually mean the option of holding the line may no longer be viable and will need to be replaced with so-called managed realignment.
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