Mexico will start a national register of mobile phone users that will include fingerprinting all customers in an effort to catch criminals who use the devices to extort money and negotiate kidnapping ransoms.

Under a new law published on Monday and due to be in force in April, mobile phone companies will have a year to build up a database of their clients, complete with fingerprints. The idea would be to match calls and messages to the phones' owners.

Hundreds of people are kidnapped in Mexico every year and the number of victims is rising sharply as drug gangs, under pressure from an army crackdown, seek new income.

Lawmakers who pushed the Bill through Congress last year say there are around 700 criminal bands in Mexico, some of them operating from prison cells, that use cell phones to extract extortion and kidnap ransom payments.

Most of Mexico's 80 million mobile phones are prepaid handsets with a given number of minutes of use that can be bought in stores without any identification. The phones can be topped up with more minutes via vendors on street corners.

The register, detailed in the government's official gazette, means new subscribers will now be fingerprinted when they buy a handset or phone contract.

The plan also requires operators to store all cell phone information such as call logs, text and voice messages, for one year. Information on users and calls will remain private and only available with court approval to track down criminals.

It was not clear whether the government would provide any funding to aid in the logistics of the register.

Billionaire Carlos Slim, who controls Mexico's No. 1 cell phone operator America Movil, said the law would be more useful if it tracked the movements of cell phone users. "What needs to be done is another type of more effective measures," Mr Slim told reporters.