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…