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.
Temporary traffic lights removed from key St Peter Port route sooner than expected
Temporary traffic lights expected to be in the Charroterie for 19 months have been removed - after just a couple of weeks.
New car for Guernsey's Meals on Wheels fleet
The Lions Club of Guernsey has funded the vehicle, which is already in service.
Major power cut switches off most of Alderney
Alderney Airport, the harbour, St Anne, and the school have all been without electricity.
Perfect evening for Guernsey's 2026 Air Display
Islanders packed St Peter Port and east coast vantage points to watch the modern and veteran aircraft perform.
The Red Arrows pull out of performing over Guernsey
The RAF display team says aircraft availability and engineering resources mean they cannot leave their Lincolnshire base overnight.
Guernsey's Ladies' Pool reopens after pollution incident
Water quality tests 'excellent' at La Vallette Ladies' Pool.
Passengers at Guernsey Airport told to expect some flight disruption
The advice is check in as usual unless your airline tells you otherwise following national flight chaos.
Guernsey flights impacted by UK air traffic control issue
Some flights between Guernsey and the UK today (8 September) have been cancelled or delayed because of an issue with UK air traffic control.
