A new court ruling means that police must obtain a warrant if they want to track people's location using their mobile phone.

The case arose when the US Department of Justice asked the federal court in the Western District of Pennsylvania to overturn a judgement that the police should be able to triangulate a person's location using their mobile phone.

"Cellphone providers store an increasing amount of sensitive data about where you are based on which cell towers your phone uses when making a call," said Kevin Bankston, senior staff attorney at the Electronic Frontier Foundation (EFF).

"Until now, the government has routinely seized these records without search warrants. This landmark ruling is hopefully only the first of many.

"Just as magistrates across the country have begun denying government requests to track cell phones in real time without warrants, based on arguments first made by EFF, so too do we hope this decision will spark new scrutiny of the government's unconstitutional seizure of stored cell phone location records. "

The case came about after an investigation into an alleged drug trafficker. The government had argued that mobile phone records were similar to credit records and therefore did not fall under Fourth Amendment rights.

Government staff also said that, because such location data was usually only accurate to within 100ft, it did not infringe on privacy.

The DoJ already accepts that it needs warrants for GPS data. It seems likely to appeal the ruling and has said that it is considering its options.