"They can not hinder freedom of residence of a citizen of Union by denying grant to ir spouse, of same sex, national of a non-member State of Union, a right of permanent residence in ir territory", he says.
The ECJ's 28 judges are not bound by the opinions of the advocate generals, but follow them in the majority of cases.
Adrian Coman and Clai Hamilton, an American, married in Brussels in 2010.
"While Member States are free to provide for marriage between persons of the same sex in their domestic legal system or not, they must fulfil their obligations under the freedom of movement of European Union citizens", he said.
EU laws guarantee citizens of member states and their family members the right to move and freely reside in any country in the bloc, subject to certain conditions, raising questions about the legal rights of same-sex spouses in countries where such unions are not legal.
Melchior Wathelet, who is a Belgian advocate in the court, said that the new rules will ensure that the rights of individuals are always upheld.
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Coman and Hamilton have been fighting for years to have their legal status as spouses officially recognised in Romania, where there is strong anti-gay sentiment.
Quoting the European Convention on Human Rights and the EU's Charter on Fundamental Rights, Wathelet called it incumbent upon member states to offer gay couples, "like heterosexuals, the opportunity of having their union recognized in law and protected by the courts". Romania's constitutional court then referred the case to the European court (ECJ).
The judge added that "the objective of protecting the traditional family can not justify discrimination on grounds of sexual orientation" and that "the concept of "spouse" within the meaning of the directive also includes spouses of the same sex".
Wathelet noted that the Netherlands and Belgium were the only two European Union member states that made marriage available to same-sex couples in 2004 when the European Union citizenship directive at issue was adopted.
The Dutch MEP Sophie in 't Veld said: "This is fantastic news and a landmark opinion for rainbow families".
A senior adviser to the European Union's top court has backed a Romanian gay man's right to have his U.S. husband live with him in Romania. They would have been compelled to do so had Mr Coman's partner been of the opposite sex. Bulgaria, Latvia, Lithuania, Poland, Romania and Slovakia still grant no legal recognition to same-sex couples.
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