Millions of cars sold by the Volkswagen Group since 1995 can be unlocked by using a £30 homemade radio to spy on key fob signals. 

The hack is effective on various models of the group's Volkswagen, Audi, Skoda and Seat brands and the company has been aware of the issue since November 2015, the BBC reported.

Although hackers can potentially crack keylocks and enter cars, Volkswagen made it clear that starting the car's engine using this method was "not possible." Newer VW models, including the Golf, Tiguan, Touran and Passat were not affected, the company said. 

Security flaws were published in an academic paper written by researchers at the University of Birmingham and German security firm Kasper & Oswald. 

Researchers reverse-engineered models' keyless entry system, and discovered a number of master cryptographic keys in the process. 

Kasper & Oswald's Timo Kasper said the discovery left them "shocked."

"Millions of keys using the same secrets - from a cryptography point of view, that's a catastrophe," he said. Researchers immediately alerted the automobile company and met with them, he said. 

News of the hack comes at a bad time for Volkswagen, which is still scrambling to repair its reputation after the company was fined billions of dollars after being caught installing cheat devices on cars to falsify emissions data. 

But owners of other car brands shouldn't sit too comfortably. Kasper & Oswald says its researchers have discovered "at least ten more, very widespread" hacking schemes affecting other brands. The company will release more information once the companies affected have been informed, it said.