An object, which is believed to be a piece of Second World War ordnance, has been discovered in the vicinity of the Reffee Buoy which is near the QE2 marina entrance.
A team of local divers discovered the ordnance during a routine dive in the area.
Guernsey Coastguard is working with Bailiwick Law Enforcement to coordinate an operation to dispose of the device.
A team of divers from the Royal Navy's explosive ordnance disposal team is expected to visit the island in the coming days to move the device away from its current location for safe disposal.
It is believed that the device had lain on the seabed undisturbed for over 70 years.
A 200-metre maritime Exclusion Zone has been established around the device and mariners should follow the instructions in the Local Notices to Mariners (LNTM 038/2020) on the Guernsey Harbours' website.
Guernsey's Harbourmaster Captain David Barker said:
"Mariners should follow the instructions of the Local Notices to Mariners and stay away from the exclusion zone which has been established. This means that, unfortunately, those who wish to enter and leave the QE2 marina will be inconvenienced for a short time, until the device has been removed. Thankfully, the discovery was made during a quiet time of year for pleasure vessel traffic, however public safety must be our first concern. Any anchoring and underwater activities are strictly prohibited in the Exclusion Zone and mariners are to only enter this area to avoid immediate danger."
No commercial vessel traffic movements into St Peter Port or St Sampson's harbours' are expected to be impacted as result of the discovery of the ordnance.