The Bailiwick of Guernsey's territorial seas are set to quadruple in size.
It comes after Guernsey, Alderney and Sark applied for an extension from three nautical miles to 12.
It means the three islands will now have more rights and control over larger area of the sea and it will bring them in-line with international norms that are enjoyed by the UK, Jersey and the Isle of Man.
It will come into force on 23 July.
The new limit of 12nm is the maximum extent permissible under international law of the sea.
In a joint statement from all three jurisdictions, they say the extension will ensure clarity about the status of the waters adjacent to the Bailiwick, especially with the UK planning to leave the EU in October.
A member of Guernsey's Policy and Resources Committee has said he is delighted about the extension.
Our islands have been working closely together to achieve this, including through the Bailiwick Council. This joint approach has enabled us to claim the marine area to the maximum limit permitted by the international law of the sea, bringing us in line with international norms and giving us more control over activities there. I believe the extension of Guernsey’s territorial sea is a significant step on a journey to ensure that the status of the seas around our shores reflects both our proud maritime history and our developing international identity. >
The Chair of Alderney's Policy and Finance Committee has said he welcomes the opportunities this extension will bring to the islands.
The enlargement of Alderney’s territorial sea means both the enhancement of economic opportunities and an increase in our marine management responsibilities and controls, as for the other jurisdictions of the Bailiwick. Our marine plan helps us to identify, understand and manage activities in the marine environment and provides evidence that the waters around Alderney are valued and well managed.