Engineers in Germany say the radio signal from every cellphone is a unique, unalterable digital fingerprint that could allow police to track a handset.

With tech-savvy criminals increasingly attempting to evade tracking by changing their cellphone's built-in ID code and regularly dumping SIM cards, the unalterable signal could still allow law enforcement officers to track a phone, they said.

The tracking method, discovered by researchers at the Technical University of Dresden, exploits the tiny variations in the quality of the various electronic components inside a phone, NewScientist.com reported Thursday.

"The radio hardware in a cellphone consists of a collection of components like power amplifiers, oscillators and signal mixers that can all introduce radio signal inaccuracies," engineer Jakob Hasse said.

As a result, the stream of data each phone broadcasts to the local cell tower contains error patterns that are unique to that phone's peculiar mix of components, he said.

In lab tests on 13 handsets, the Dresden team was able to identify the source handset with an accuracy of 97.6 percent.

"Our method does not send anything to the mobile phones. It works completely passively and just listens to the ongoing transmissions of a mobile phone -- it cannot be detected," Hasse said.

The new method is an attractive alternative to existing tracking techniques, experts said.

"Serious criminals are extremely adept in using single-use phones and dumping SIM cards so new capabilities like this would certainly help law enforcement," Nick Furneaux of forensics security company CSItech in Britain said.