Top tech giants, including Apple, Google and Microsoft have slammed a proposal by the UK intelligence agency GCHQ to snoop on users' encrypted chats.

In an open letter published on Lawfare, a group of 47 tech firms on Thursday criticised the move, asking the agency to abandon its plans for a so-called "ghost protocol".

The signatories of the open letter argued that the proposal by the UK cyber security agency would require two major changes to their systems which would, in turn, compromise the users' privacy.

"First, it would require service providers to surreptitiously inject a new public key into a conversation in response to government demand.

"This would turn a two-way conversation into a group chat where the government is the additional participant, or add a secret government participant to an existing group chat," read the open letter.

"Second, in order to ensure the government is added to the conversation in secret, GCHQ's proposal would require messaging apps, service providers, and operating systems to change their software so that it would 1) change the encryption schemes used, and/or 2) mislead users by suppressing the notifications that routinely appear when a new communicant joins a chat."

The proposal from GCHQ was first published last November as part of a series of essays, reports The Verge.

In the essay, two senior British intelligence officials argued that law enforcement should be added as a "ghost" participant in every encrypted messaging conversation, said the report.

In 2015, Apple had taken a tough stand against the US Federal Bureau of Investigation (FBI) when asked to unlock an iPhone of a user involved in a terror attack in Bernardino, California.