Mobile phone firms are being overwhelmed by police requests for information about suspects' calls, the companies have complained.

Almost half a million inquiries are made to the firms every year by police and customs officers, the BBC has learned.

The cost of the inquiries to both police and the industry has been escalating, and firms have begun charging a fee for every request.

Information on where or when a mobile phone call was made by a suspect can link them to a crime - or exonerate them.

The uncle of Essex schoolgirl Danielle Jones was on Thursday convicted of her murder, partly because of evidence he had sent himself a text message from her phone after she had vanished.

In April, two 16-year-old brothers were cleared of murdering 10-year-old Damilola Taylor after a judge ruled they could not have run from where Damilola sustained his fatal injury, to an area where they made mobile calls shortly afterwards.

Police frustration

Gregory Smith, a consultant forensic engineer, said the "radio footprints" left by criminals using mobiles were vital to many criminal investigations.

"This is just another branch of the forensic science area ... like pathology, medicinal and accounting," he said.

"If a person makes a mobile call, potentially while involved in commission of a criminal act, it is possible to determine from where the radio footprint would have been made."

But police say they are frustrated because the information is not always available. Records are sometimes held for only six months.

A year ago the government proposed a code of conduct in which firms would keep records for at least 12 months - to enable it to carry out checks in its fight against terrorism.

But negotiations have not gone well, and there has been no agreement with the mobile phone companies.

The Home Office said discussions were continuing.