Apple Inc., Google LLC, Microsoft Corp. and WhatsApp Inc. joined civil society groups, trade groups and security experts in urging British intelligence agency GCHQ to scrap its "ghost proposal" that would facilitate eavesdropping on encrypted messages.

The tech companies said in an open letter that the idea "poses serious threats to cybersecurity and fundamental human rights, including privacy and free expression." They added that the proposal would make messaging platforms open to abuse or misuse.

It was put forward by Ian Levy, technical director at the National Cyber Security Centre, and GCHQ's technical director for cryptanalysis, Crispin Robinson.

Under the proposal, service providers would be required to "silently add a law enforcement participant" to a group chat or call. The solution is "no more intrusive than the virtual crocodile clips" used in wiretapping nonencrypted communications, Levy and Robinson wrote on Lawfare.

The open letter, however, argued that such a mechanism would require changes that threaten user privacy. These include secretly adding a new public key and changing encryption schemes used in a conversation.

In response, Levy said the proposal was intended to enable an open discussion on the matter, London's The Guardian reported.

GCHQ has previously drawn flak for allegedly spying on internet users in Britain. Former U.S. government security contractor Edward Snowden had revealed the U.K.'s work with the U.S. National Security Agency to covertly gather and share data from tech companies under the PRISM surveillance program.

WhatsApp is a Facebook Inc.-owned messaging service. Alphabet Inc. owns Google.