The backdoor may have existed for decades, potentially exposing a wealth of sensitive information transmitted across them, according to the researchers.

For those not in the know, TErrestrial Trunked RAdio (TETRA) standard has been in play for more than 20 years as a form of super-secure digital communication.

Jos Wetzels, one of the researchers from cybersecurity firm Midnight Blue insists that part of the standard  encryption algorithm called TEA1 is a backdoor. Although those who designed the standard said it was designed for export controls which determine the strength of encryption.

Wetzwels points out that radios with traffic can be decrypted using consumer hardware like an ordinary laptop in under a minute.

"There's no other way in which this can function than that this is an intentional backdoor," he said.

The boffins found other, multiple vulnerabilities across TETRA that could allow historical decryption of communications and deanonymisation.