The gadget makes a copy of the key’s signal when the owner unlocks their vehicle. Once it is unattended, thieves can use it to break in.
Called HackRF One, the device retails at £257 and is marketed for the “development of modern and next generation radio technologies”.
The box adds that “you are responsible for using your HackRF legally”.
But Andrew Miller, chief technical officer at Thatcham Research, a motor insurers’ centre, told the Mail on Sunday: “Most of these technologies are designed for only one purpose, which is to break into a car.
“It tends to be organised crime which uses these devices and the problem is only going to get worse. The Government needs to review the availability of these items.”
More than 6,000 cars and vans were stolen in London in 2015 by thieves using devices such as signal grabbers.
Manufacturers have incorporated encryption in an attempt to thwart such technology, but HackRF One uses a jammer to circumvent the security layer.
A spokesman for eBay said: “The device is widely available and is advertised as having a broad range of uses. We have not been advised of any restrictions on its sale.”
Amazon declined to comment.
Michael Ossman, founder of Great Scott Gadgets, the manufacturer of Hack RF One, told the Mail on Sunday: ‘We encourage auto makers and automobile owners to use test equipment such as HackRF One to test their own vehicles as permitted by law.”
Researchers at a Chinese security firm demonstrated earlier this month how easy it is to break into cars. Using a pair of gadgets costing as little as $22, the team showed that it was possible to unlock a car and drive it away in minutes.