The British government's plan to "take back control" of its waters after leaving the EU is about to be challenged by a claim from Denmark that its fishermen have a historical right to access to the seas around Britain dating back to the 1400s.
Officials in Copenhagen have mined the archives to build a legal case that could potentially be fought in the international court of justice in the Hague, although officials hasten to say that this is not their intention.
Denmark is seeking a Brexit deal that recognises the right of its fleet to continue to exploit a hundred shared stocks of species such as cod, herring, mackerel, plaice and sand eel.
Officials say 40% of Danish fishermen's annual take is from waters within the 200-nautical mile exclusive economic zone around Britain, over which the UK may seek to impose greater control when it leaves the EU. Some of Denmark's coastal communities are almost entirely economically dependent on access to UK waters.
The development suggests that leaving the EU will not reap the dividends promised by prominent leave campaigners, including the foreign secretary, Boris Johnson, who has claimed that the union's "crazy" common fisheries policy has halved UK employment in the industry.
The Danish position is likely to be mirrored by the seven other member states who will be affected if the UK seeks to limit access to its waters to EU fleets after 2019, it is understood.