Middle-class families in Mexico are having tiny transmitters implanted under their skin so that satellites can track them if they are kidnapped.

Sales of the device have jumped by 13 per cent this year after kidnappings surged by almost 40 per cent in the country between 2004 and 2007.

The crystal-encased chip, which is the size and shape of a grain of rice, is injected into clients' bodies with a syringe.

A transmitter in the chip sends radio signals to a device, carried by the client, with a global positioning system in it, say makers Xega. A satellite can then pinpoint the kidnap victim's location.

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Kidnap appeal: A victim's family used a giant billboard depicting a member of the gang which took their son in Mexico City

Detractors say the chips, which cost £2,000 plus an annual fee of £1,100, give a false sense of security.

Mexico ranks with conflict zones such as Iraq and Colombia as among the worst countries for abductions.

One client, Cristina, 28, who did not want to give her last name, said: 'It's not like we are wealthy, but they'll kidnap you for a watch. Everyone is living in fear.'

The recent kidnapping and murder of Fernando Marti, 14, the son of a well-known businessman, sparked an outcry in a country already hardened to crime.

Target: Businessman Alejandro Marti, centre, whose 14-year-old son Fernando was kidnapped

Most kidnappings in Mexico go unreported, many of them cases of 'express kidnapping' where the victim is grabbed and forced to withdraw money from automatic cash machines.

Official statistics show 751 kidnappings in the country last year, but the independent crime research institute ICESI says the number could have exceeded 7,000.

Xega, based in the central Mexican city of Quererato, designed global positioning systems to track stolen vehicles until a company owner was kidnapped in broad daylight in 2001.

Frustrated by his powerlessness to call for help, the company adapted the technology to track stolen people.

Most people get the chips injected into their arms between the skin and muscle where they cannot be seen.

Customers who fear they are being kidnapped press a panic button on an external device to alert Xega, which then calls the police.

Chips are down: AGPS satellite like this one is used to pinpoint where potential hostages are

'Before, they only kidnapped key, well-known economically successful people like industrialists and landowners. Now they are kidnapping people from the middle class,' said Sergio Galvan, Xega's commercial director.

Katherine Albrecht, a U.S. consumer privacy activist, says the chip is a flashy, overpriced gadget that only identifies a person and cannot locate someone without another, bigger GPS device that kidnappers can easily find and destroy.

She said fear of kidnapping was driving well-off Mexicans to buy a technology that had yet to prove useful.

'They are a prime target because they've got money and they've got a worry and you can combine those two and offer them a false sense of security which is exactly what this is,' she said.

President Felipe Calderon has come under heavy pressure to stamp out violent crime. He hosted a meeting on Thursday of security chiefs and state governors.

Outside of Mexico, U.S. company VeriChip Corp uses similar radio-wave technology to identify patients in critical condition at hospitals or find elderly people who wander away from their homes.

Xega sees kidnapping as a growth industry and is planning to expand its services next year to Brazil, Colombia and Venezuela.