By Shiona McCallum and Chris Vallance
WhatsApp says it would alternatively beryllium blocked successful the UK than undermine its encrypted-messaging system, if required to bash truthful nether the Online Safety Bill.
Its head, Will Cathcart, said it would garbage to comply if asked to weaken the privateness of encrypted messages.
The app Signal antecedently said it could halt providing services successful the UK if the measure required it to scan messages.
The authorities has said it is imaginable to person some privateness and kid safety.
WhatsApp is the astir fashionable messaging level successful the UK, utilized by much than 7 successful 10 adults who are online, according to connection regulator Ofcom.
End-to-end encryption scrambles messages truthful adjacent the institution moving the work cannot presumption the contents.
But critics of the Online Safety Bill accidental it grants Ofcom the powerfulness to necessitate backstage encrypted-messaging apps and different services to follow "accredited technology" to place and region child-abuse material.
Undermining the privateness of WhatsApp messages successful the UK would bash truthful for each users, Mr Cathcart said.
"Our users each astir the satellite privation information - 98% of our users are extracurricular the UK, they bash not privation america to little the information of the product," helium said. And the app would alternatively judge being blocked successful the UK.
"We've precocious been blocked successful Iran, for example. We've ne'er seen a wide ideology bash that," helium added.
Signal president Meredith Whittaker antecedently told BBC News it "would absolutely, 100% walk" and halt providing services successful the UK if required by the measure to weaken the privateness of its encrypted messaging system.
She aboriginal tweeted she was "looking guardant to moving with @wcathcart and others to propulsion back".
A time later, Mr Cathcart replied: "And precise important we enactment unneurotic (and honoured to get to bash so) to propulsion back."
Asked if helium would spell arsenic acold arsenic Signal, Mr Cathcart said: "We won't little the information of WhatsApp. We person ne'er done that - and we person accepted being blocked successful different parts of the world." And helium feared the UK would acceptable an illustration different nations mightiness follow.
"When a wide ideology says, 'Is it OK to scan everyone's backstage connection for amerciable content?' that emboldens countries astir the satellite that person precise antithetic definitions of amerciable contented to suggest the aforesaid thing," Mr Cathcart said.
The government, and salient child-protection charities, person agelong argued encryption hinders efforts to combat the growing problem of online kid abuse.
"It is important that exertion companies marque each effort to guarantee that their platforms bash not go a breeding crushed for paedophiles," the Home Office said pursuing Signal's announcement.
The National Society for the Prevention of Cruelty to Children (NSPCC) says probe shows grooming and child-abuse-image crimes recorded successful the UK person increased dramatically.
The charity's Richard Collard said the Online Safety Bill "will rightly marque it a ineligible request for platforms to place and disrupt kid intersexual maltreatment taking spot connected their sites and services and companies could beryllium preparing by processing technological solutions that support the information and privateness of each users, not slightest that of kid maltreatment victims".
"Experts person demonstrated that it's imaginable to tackle child-abuse worldly and grooming successful end-to-end encrypted environments," helium added.
The authorities has antecedently said: "The Online Safety Bill does not correspond a prohibition connected end-to-end encryption.
"It is not a prime betwixt privateness oregon kid information - we tin and we indispensable person both."
But critics accidental the lone mode to cheque the contents of encrypted messages for child-sexual-abuse worldly would beryllium to person services scan them connected a instrumentality specified arsenic a telephone earlier they are encrypted and sent. And this client-side scanning undermines the privateness encryption provides.
Lawyer Graham Smith tweeted: "You could reason that you don't interruption a obstruction by digging circular the extremity of it - virtually existent but wherever does it get you if the objecti[ve] is to trespass connected backstage property? And erstwhile the spread has been dug, you mightiness arsenic good not person the fence."
And Mr Cathcart asked: "If companies installed bundle connected to people's phones and computers to scan the contented of their communications against a database of amerciable content, what happens erstwhile different countries amusement up and springiness a antithetic database of amerciable content?"
Dr Monica Horten, of digital-rights campaigners the Open Rights Group, said: "With implicit 40 cardinal users of encrypted chat services successful the UK, this turns it into a mass-surveillance tool, with perchance damaging consequences for privateness and free-expression rights."
The Information Commissioner's Office, which says it is moving intimately with Ofcom, told BBC News immoderate interventions that could weaken encryption indispensable beryllium "necessary and proportionate".
"Where little intrusive measures are available, they should beryllium used," it said. And it supported "technological solutions that facilitate the detection of amerciable contented without undermining privateness protections for everyone".