Messaging firms including WhatsApp have voiced their alarm over Government plans forcing them to release private messages to stop child sex abuse.
Ministers are pushing for regulator Ofcom to be allowed to access encrypted messages in "limited" circumstances.
The Government says the power, included in the long-awaited Online Safety Bill, would only be used to tackle "unprecedented" levels of child sexual abuse.
But bosses from Signal and Element have also warned the move could open the door to "indiscriminate surveillance" of personal messages.
This could lead to "routine" surveillance of personal messages which could be used to target journalists, human rights groups and politicians, they said.
The Home Office meanwhile says tech firms have a "moral duty" to allow law enforcement agencies to access messages if paedophiles can be identified and stopped.
In an open letter, tech firms have called for an "urgent rethink" of the bill.
WhatsApp boss Will Cathcart said: "Private messages are private.
"We oppose proposals to scan people's private messages, and we're proud to stand with other apps to defend encryption and your right to privacy."
Meanwhile Element - which is based in the UK - has warned that it could be forced to move overseas if the "outright dangerous" legislation is passed in its current form.
Chief executive Matthew Hodgson warned that rogue states would seek to exploit any access into encrypted systems introduced by the legislation.
He stated: "The UK wants its own special access into end-to-end encrypted systems."
"Bad actors don't play by the rules. Rogue nation states, terrorists, and criminals will target that access with every resource they have.
"OSB is outright dangerous. It's the cyber equivalent of Britain decommissioning its nuclear deterrent."
In a joint letter tech bosses warned: "As currently drafted, the Bill could break end-to-end encryption, opening the door to routine, general and indiscriminate surveillance of personal messages of friends, family members, employees, executives, journalists, human rights activists and even politicians themselves, which would fundamentally undermine everyone's ability to communicate securely."
A Home Office spokesman said: "We support strong encryption, but this cannot come at the cost of public safety.
"Tech companies have a moral duty to ensure they are not blinding themselves and law enforcement to the unprecedented levels of child sexual abuse on their platforms.
"The Online Safety Bill in no way represents a ban on end-to-end encryption, nor will it require services to weaken encryption.
"Where it is the only effective, proportionate and necessary action available, Ofcom will be able to direct platforms to use accredited technology, or make best endeavours to develop new technology, to accurately identify child sexual abuse content, so it can be taken down and the despicable predators brought to justice."
And Downing Street has defended the measures.
The PM's official spokesman said the Government "will not introduce routine scanning of private communication".
He said: "It is being developed to ensure it has the requisite safeguards so it doesn't weaken, by default, end-to-end encryption, it is a targeted power to be used only when necessary and when other measures cannot be used."
