Brexit has sparked its first territorial dispute - reigniting an ancient row over the ownership of Lough Foyle.
Claims over the vast estuary between Co Derry in Northern Ireland and Co Donegal in the Republic of Ireland have been made since the island was partitioned almost a century ago.
After the Good Friday Agreement peace deal, a cross-border body called the Loughs Agency was handed responsibility for the waters, a key strategic naval base during the Second World War.
But in the wake of the UK’s vote to leave the European Union, Northern Ireland Secretary of State James Brokenshire has reasserted London’s claim over the entire lough.
In response, Dublin has issued a fresh declaration saying it does not accept the claim and does not see Lough Foyle’s disputed ownership being put on the table as part of the Brexit negotiations.
Mr Brokenshire was asked in a parliamentary question how fishing rights will be decided in both Lough Foyle and Carlingford Lough - which also straddles both jurisdictions - after the UK leaves the EU.
The Conservative minister said London is committed to withdrawing from the EU Common Fisheries Policy and putting a new fisheries regime in place.
But no actual decisions have yet been taken, he said, adding that the UK was bound by international law.
Asked specifically about Lough Foyle he added: “The Government’s position remains that the whole of Lough Foyle is within the UK.”
Dublin’s Department of Foreign Affairs swiftly rejected the claim.
“Ireland has never accepted the UK’s claim to the whole of Lough Foyle,” it said in a statement.
Dublin said both governments agreed to try and resolve the ongoing row over both Lough Foyle and Carlingford Lough during talks in 2011 between the then minister for foreign affairs and the British foreign secretary.
“Since that time a series of meetings have taken place at official level between the Foreign and Commonwealth Office and the Department of Foreign Affairs and Trade,” the statement added.
“The issues involved are complex and involve a range of different actors, including the Crown Estates.
“This is not something we currently envisage as forming part of the negotiations around the UK’s departure from the EU.”
Sinn Fein Senator Padraig Mac Lochlainn has branded Mr Brokenshire’s remarks as “arrogant and provocative”.
“The Loughs Agency tasked with responsibility for managing Lough Foyle by both governments has been repeatedly calling for a resolution so that the real tourism and fisheries potential of the Lough can be fully realised,” he added.