A hole has appeared behind the end of the German wall at the eastern side of L'Ancresse, near the slipway.
The States of Guernsey has fenced the area off, and work will start next week to fill it.
Under the latest States resolution, Environment and Infrastructure are to keep repairing the crumbling World War Two defence work for a number of years.
A £150,000 repair plan was agreed in April.
Members of the public are being asked to avoid the area and not pass the barriers.
Jersey and Guernsey's Chambers of Commerce saddened by Blue Islands' collapse
Both Jersey and Guernsey's Chambers of Commerce have expressed 'sadness' at Blue Islands' decision to suspend flights to and from the Channel Islands.
Aurigny announces inter-islands flights
Aurigny has announced it is beginning flights between Jersey and Guernsey following the loss of Blue Islands services.
Loganair steps in to take some Blue Islands routes
The Scottish regional airline Logainair is stepping in to take over some routes operated by Blue Islands which has suspended operations.
Blue Islands cancels upcoming flights as airline ceases trading
Blue Islands has operated its final flights after it failed to get a last-minute lifeline from Jersey's government.
Simon Calder: 'To lose one airline is unfortunate, two looks like carelessness'
Travel journalist Simon Calder says there are questions to be answered on whether regional airlines receive enough support, after Blue Islands and Eastern Airways collapsed a few weeks apart.
Guernsey creative David Robilliard awarded Blue Plaque
A Blue Plaque in Market Square hopes to inform more locals about the life and work of David Robilliard.
Guernsey Conservation Volunteers win King's Award
The King's Award recognises three decades of volunteer effort to remove invasive plants from the coast and countryside.
Winners pick up awards for providing Guernsey's best customer service
Winners and nominees pack the OGH for the 2025 Norman Piette Customer Service Awards.
