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.
The Chair of Alderney's Policy and Finance Committee has said he welcomes the opportunities this extension will bring to the islands.