In a letter sent to GCHQ, the coalition outlined its concerns that the spy agency's proposal posed a serious threat to cyber security and human rights.
Among the companies which signed the letter were Apple, Google, Microsoft and WhatsApp.
The proposal from GCHQ was made last year by its director Ian Levy, and involved "silently adding a law enforcement participant to a group chat or call", according to an article written by Levy and his colleague, Crispin Robinson.
But this required two important changes: service providers would have to quietly inject a new public key into a conversation if a government made a demand for it. This meant the government would become an additional participant in the conversation.
Secondly, the letter said this meant the software being used would have to be changed, that it would alter the encryption schemes used and mislead users by suppressing the notifications that routinely appear when a new participant joins a chat.
"The GCHQ’s ghost proposal creates serious threats to digital security: if implemented, it will undermine the authentication process that enables users to verify that they are communicating with the right people, introduce potential unintentional vulnerabilities, and increase risks that communications systems could be abused or misused," the signatories to the letter said.
"These cyber security risks mean that users cannot trust that their communications are secure, as users would no longer be able to trust that they know who is on the other end of their communications, thereby posing threats to fundamental human rights, including privacy and free expression. Further, systems would be subject to new potential vulnerabilities and risks of abuse."