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.
BA's new Heathrow route brings first time visitors to Guernsey
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CIAS Backs mental health drive as missing person searches rise
A mental health awareness campaign called "We See You" is launching in the Channel Islands.
Abandoned vehicles removed from Guernsey car park
Two abandoned vehicles described as an "eyesore" have been removed from public property at no cost to the taxpayer.
21st century revival for The Wombles
The eco-friendly fictional creatures, The Wombles, could return to television.
World Cup excitement builds in Channel Islands
The World Cup excitement is going up a notch in the Channel Islands today, as England and Portugal play for the first time.
Skirmish between British yacht and Russian warship near Alderney
A Russian frigate fired small arms warning shots at the private yacht as they passed north east of Alderney.
Three banned from Jersey and Guernsey FA matches following Muratti violence
Three people have been banned from Jersey and Guernsey FA matches following violence at last month's Muratti.
'World-class' Guernsey golf course to open this summer
La Grande Mare Golf Course has been extended and reconfigured over seven years.
