Luxembourg - Unaccompanied minors who arrive in the European Union and turn 18 during their asylum process can still be joined by their families, the bloc's top court ruled Thursday, in a decision that could have consequences for member states including Germany.

Migration and asylum issues are politically sensitive in the EU, where populist parties have seized on the issue following the arrival of more than 1 million migrants to the bloc in recent years.

Court: Refugee's date of arrival counts

The case relates to an Eritrean who arrived in the Netherlands and applied for asylum in early 2014, aged 17. She turned 18 in June, four months before being granted a residence permit.

At the end of 2014, an application was submitted on her behalf for her parents and three brothers to join her, but the Dutch authorities turned it down, arguing that she was no longer a minor and therefore not entitled to family reunification.

The decision was appealed before a Dutch court, which turned to the European Court of Justice for advice.

The Luxembourg-based judges ruled Thursday that family reunification claims must be based on a refugee's date of arrival, as any other system would not guarantee equal treatment or legal certainty.

Germany, the EU's largest member state and home to most asylum seekers in the bloc, may have to change its rules on family reunification following the verdict.

At present, the parents of a refugee are only entitled to join their child before it has reached the age of 18, according to the Foreign Ministry in Berlin.

A visa can therefore only be granted as long as the child is a minor, according to the ministry, which was not immediately available for a reaction.

The ECJ argued that, if family reunification requests hinge on the date that asylum was granted, this would make the decision dependent on how quickly a member state processed asylum requests - for example in cases where the child turns 18 during this period.

This would open the possibility of different outcomes for two minors arriving in the EU at the same time, the court said in a statement.

However, the family reunification application must be made within a "reasonable" period of three months after being granted refugee status, the judges said.