The case before the court involved a Romanian activist, Adrian Coman, and his American husband, Claibourn Robert Hamilton, who were married in Belgium in 2010.

When they tried to move to Romania a few years later, the country denied Hamilton spousal residency rights, as it does not recognise same-sex marriage.

The couple, who now live in the United States, filed suit in Romania in 2013.

The Court of Justice took up the case in November 2016, after Romania's Constitutional Court requested an interpretation of EU law.

For LGBT rights groups, the verdict has been a long time coming.

Loading

"In 2004, when the freedom of movement directive was adopted, the term spouse was deliberately left vague. Since then, same-sex couples have been left in a legal limbo, in uncertainty," said Katrin Hugendubel, the advocacy director at ILGA-Europe, a Brussels-based group that promotes gay and transgender rights.

With this case, she added, the court has clarified that all EU member states need to recognize marriages carried out in other member states and that all European citizens and their spouses have "full freedom of movement, which is one of the four fundamentals of the European Union".

Romania decriminalised homosexuality only in 2001, and it joined the European Union in 2007.

The court's verdict came at an important moment, with a referendum expected in the coming months on whether to change the constitutional definition of marriage to specify that it must be a union between a man and a woman.

The referendum, which came about after a 2016 petition gathered 3 million signatures, would make it harder for the country to legalise same-sex marriages in the future.

"This is a worrying time for all of us who want to live in a more inclusive society," said Florin Buhuceanu, president of the Romanian advocacy group Accept.

"We are living in the 21st century, in the EU," he added. "It is the right time to start recognising these families as families."

Thirteen of the EU's 28 member states currently allow same-sex marriage, while a further nine allow civil unions or something similar. Bulgaria, Latvia, Lithuania, Poland, Romania and Slovakia have neither.

Loading

Robert Wintemute, a professor of human rights law at King's College London, said that the principle that spouses include same-sex partners will now be immediately binding for all courts in the bloc's current member states and for those in any countries that join later.

It could also put pressure on the six member states without legal recognition of same-sex unions to introduce some form of legislation, he added.

The couple's lawyer, Iustina Ionescu, described the case as "not just about same-sex marriage but about what the EU stands for - dignity, equality, respect for basic freedoms for all of us".

And while the ruling is limited to same-sex couples married in a European member state, Ionescu believes the verdict suggests that any future case brought by a couple married outside the bloc would be successful.

Coman said that he had dreamed about this moment for a long time, adding: "I can barely believe it".

Still, he said, he had been optimistic about the eventual verdict. "Come on, it's 2018, it's one of the core freedoms of the EU," he said. "But we didn't expect it to take this long."