WhatsApp messages could be read by Facebook and government agencies, according to a researcher who has discovered a security hole in the app.

Fears over users privacy were sparked after Tobias Boelter, a security researcher at the University of California, Berkeley, discovered a backdoor in WhatsApp's method of end-to-end encryption. The encryption technology was added last year to ensure that no one - including the company - can read a user's messages other than them.

It means the company could intercept messages sent to phones that aren't connected to the internet and forward them on to a separate device without the sender or receiver knowing. The messages could still be sent to the intended device, leaving users that don't have security notifications switched on completely unaware.

"If WhatsApp was asked by a government agency to disclose its messaging records it can effectively grant access due to the change in keys," Boelter told the Guardian.