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.
Renaissance 'masterpieces' on display in Guernsey
"Remarkable artworks" from the Renaissance are currently on display in St Peter's Port.
Guernsey's airline reacts to reduced demand because of Iran attacks
Aurigny is rescheduling some flights between mid April and June and will levy a £2 fuel surcharge per sector.
Survey reveals scale of dissatisfaction with off-island travel
A new survey reveals just how dissatisfied Channel Islanders are with the cost and reliability of travelling off-island.
Two youngsters questioned by Guernsey Police after fire at Leale's Yard
Fire crews spent one and a half hours putting out the blaze in a derelict building.
Prominent Guernsey consultant cardiologist suspended by the GMC
The MSG says Dr Dean Patterson is no longer a partner but that he is fighting his temporary medical suspension.
Guernsey FC features on Premier League social media
A video exploring the formation of GFC and Victoria Park has been posted by the Premier League.
Law banning under-18s enter Guernsey vape shops labelled 'hypocritical'
A Guernsey business wants to meet with Health and Social Care before next week's new law bans under-18s from entering vape shops unaccompanied.
Channel Island iPhone users struggle with age verification update
Islanders in Guernsey and Jersey are having issues verifying their age, which is required by the new iPhone update.
