The European Court of Justice (ECJ) has been asked to consider whether Britons living in the 27 other member states can retain their rights as EU citizens after Brexit.
The Dutch courts have referred the case, which could have implications for a million Britons living in other EU countries, to the ECJ.
Campaigners have suggested that the case could also protect the EU citizenship of UK residents born before Brexit.
The judge in Amsterdam said in a written ruling that “there has to be more clarity about the consequences of Brexit for EU citizenship”.
The UK Government insisted that its deal with Brussels meant expats could “continue living their lives broadly as they do now” after Brexit.
UK citizens who live in the Netherlands went to the court in a bid to retain their EU citizenship rights after Britain leaves the bloc in March 2019.
Christiaan Alberdingk Thijm, the lawyer who represented the Britons in the Amsterdam court, welcomed the ruling.
He said: “Theresa May famously said ‘Brexit means Brexit’, but the Brits currently living on the continent have no idea what that means for them.
“Are you an EU citizen for life, or can your citizenship be taken away from you? That is the fundamental question that will be put forward to the European Court.”
British national Stephen Huyton, who has lived in the Netherlands for 24 years, said he was delighted with the decision.
“However, this is but the first step to clarity about what Brexit means for our EU citizenship,” he said.
Leading barrister Jolyon Maugham QC, who supported the Netherlands case, said the decision of the ECJ – also known as the Court of Justice of the EU (CJEU) – could even have implications for British nationals in the UK.
“The case – if determined by the CJEU in our favour – could lead not only to the retention of EU citizenship for UK residents living in the EU,” he said.
“It could lead to all born before Brexit – even if living here in the UK – retaining the boon of EU citizenship.”
The issues referred to the ECJ are whether Brexit automatically leads to the loss of the EU citizenship and its associated rights and, if it does not, whether conditions or restrictions should be imposed on those rights.
A Department for Exiting the EU spokesman said: “We have secured a deal that will safeguard the rights of UK nationals living in the European Union, so that they can continue living their lives broadly as they do now.”