The bug arises because of the way WhatsApp encrypts the messages sent via its service.

Security expert Thomas Boelter found that eavesdropping was possible when circumstances called for encryption keys to be reissued.

Mr Boelter told WhatsApp owner Facebook about the issue in April 2016 but it said it was not working on a fix.

The response he received said that what he had discovered was expected behaviour.

Privacy campaigners claimed in The Guardian newspaper that the bug was a "huge threat" to freedom of speech because it could be used by governments or law enforcement agencies to spy on people who thought they were communicating securely.

In a statement reacting to media stories about the research, WhatsApp said the bug was not a "backdoor" intentionally placed in its code that allowed governments to make the firm decrypt messages.

"This claim is false," it said. "WhatsApp does not give governments a 'backdoor' into its systems and would fight any government request to create a backdoor."
 

Bad coding

The bug crops up in situations when encryption keys used to scramble messages have to be reissued and resent.

Mr Boelter found that, in certain circumstances, attackers can pose as the recipient of a message and force WhatsApp to reissue keys for scrambling information.

Sophisticated manipulation of this system would let attackers intercept and read messages, said Mr Boelter.

Zack Whittaker, security editor at ZDNet, said it was a "stupid and big bug" but played down its seriousness.

The problem was "limited" in its scope, he said, adding that it probably emerged because of "bad coding or a favour to good user experience".

In its statement, WhatsApp said it had taken a design decision to implement the re-issuing of keys in this way to preserve millions of messages that would otherwise be lost.

Cryptographer Frederic Jacobs said anyone worried about falling victim to the bug could adjust security settings on the app to warn them if encryption keys were being changed.