A German law requiring telecoms companies to retain customer data is a breach of EU legislation, a European court ruled Tuesday, prompting the justice minister to vow an overhaul of the rules. 

Firms Telekom Deutschland and SpaceNet took action in the German courts challenging the law that obliged telecoms companies to retain customers’ traffic and location data for several weeks. 

The case headed to the European Court of Justice (ECJ) in Luxembourg, which ruled against the German legislation.

"EU law precludes the general and indiscriminate retention of traffic and location data," the court said in a statement, confirming its previous judgements on the issue.

The Federal Administrative Court, one of Germany’s top courts, had argued there was a limited possibility of conclusions being drawn about people’s private lives from the data, and sufficient safeguards were in place. 

But the ECJ said the German legislation -- which required traffic data to be retained for 10 weeks, and location for four -- applies to a "very broad set" of information. 

It "may allow very precise conclusions to be drawn concerning the private lives of the persons whose data are retained... and, in particular, enable a profile of those persons to be established."

The stated aim of the law was to prosecute serious criminal offences or prevent specific risks to national security, but the court said that such measures were not permitted on a "preventative basis".

However, it said that in cases where an EU state faces a "serious threat to national security" that is "genuine and present", telecoms providers can be ordered to retain data. 

Such an instruction must be subject to review and can only be in place for a period deemed necessary. 

Following the announcement, Justice Minister Marco Buschmann hailed a "good day for civil rights".

"We will now, swiftly and definitively, remove data retention without cause from the law," the minister wrote on Twitter.

Data privacy is a sensitive issue in Germany, and its courts have in the past issued rulings aiming to limit security services access to people’s data.

Buschmann is from the liberal FDP party, which has made data protection a key plank of its policies.