A three-person panel at the Permanent Court of Arbitration in the Hague will hear arguments from both sides for three days on whether a British ban on fishing sand eels in its North Sea waters breaches the EU-UK Trade and Co-operation Agreement (TCA).
The three legal experts from France, New Zealand, and South Africa have until late April to deliver a final ruling.
In financial terms, the case is trifling. Britain puts the revenue loss for non-UK vessels at STG45 million in a worst-case scenario.
Politically, it could prove awkward.
The case is due to be heard by the Permanent Court of Arbitration in The Hague. (EPA PHOTO)
The three-person tribunal can uphold the ban or rule it breaches the TCA, allowing the EU to retaliate if it is not lifted.
That would upset environmentalists and Brexiteers, who could pressure Britain's Labour government not to yield, a stance that would hamper hopes for better ties with the bloc.
British Prime Minister Keir Starmer is due to join EU leaders for a retreat and discuss closer defence ties to counter Russia and to respond to US President Donald Trump's demand that NATO members increase military spending.
Britain is also expected to address other areas in the coming months, such as a veterinary agreement with the EU that would ease agricultural and food trade.
It says the fishing ban is necessary, reflecting scientific advice on the resilience of the species and the role it plays in the food chain of predators  - larger fish, marine mammals, and seabirds such as puffins.
British boats do not fish for the small eel-like fish, but sand eels are caught by Danish fleets and used as animal feed and a source of oil.
The EU contends the ban is discriminatory, disproportionate, and not based on the best available scientific evidence. It says the sand eel fishing prohibition constitutes an unjustified restriction on the access of EU vessels to UK waters granted by the TCA.
