The move appears to put Meta at odds with the government again, which wants to strengthen protections online, in particular for children - but companies like Meta warn changes to the law to improve online safety could undermine the privacy of messages.
WhatsApp users will soon be able to lock and hide conversations, thanks to a new feature.
Chat Lock will remove a chat thread from the app's regular onscreen inbox and place it into a new folder that can only be opened by a password or biometric, such as facial recognition or a fingerprint.
Calling it "one more layer of security", Meta - WhatsApp's parent company - added Chat Lock will protect "your most intimate conversations" and hide notifications from them.
It's the latest in a list of growing features on the globally-used, encrypted messaging service which puts it at odds with the UK government's online safety bill.
As part of its privacy package, Meta allows WhatsApp users to encrypt their backups, block the ability to screenshot and make their messages disappear automatically.Mark Zuckerberg
, Meta's chief executive officer, confirmed the new feature in a Facebook
He said: "New locked chats in WhatsApp make your conversations more private. They're hidden in a password protected folder and notifications won't show sender or message content."
Meta has criticised the Online Safety Bill, along with other companies.
It says the law change would undermine end-to-end encryption - a level of security for messages which means nobody other than the users involved in the conversation is able to see its content.
The company has previously warned it would sooner see British users stopped from using its services than risk compromising their privacy.
But a government spokesperson insisted the bill "will not require companies to break end-to-end encryption or routinely monitor private communications".
"Some have characterised this as a binary choice between privacy and safety: this is wrong," the spokesperson added.
They continued: "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 criminality on their platforms.
"As a result of our pro-innovation approach, we are confident technology can support the implementation of end-to-end encryption in such a way that can protect children from abuse online, while respecting user privacy."
Some charities, including the NSPCC, also say they support the aims of the bill and surveys suggest it has the backing of large numbers of British adults.
However, UK-based messaging platform Element, used by the likes of the Ministry of Defence, US Marine Corps, and Ukraine's armed forces, claimed the bill was "outright dangerous" and would weaken national security.
Element's chief executive Matthew Hodgson said: "Bad actors don't play by the rules. Rogue nation states, terrorists, and criminals will target that access with every resource they have."
Mr Hodgson added: "It's a shock to see the UK, a country that symbolises democracy and freedom, introducing routine mass surveillance and fundamentally undermining encryption.
"Bad actors will simply continue to use existing unregulated apps - and good actors using compliant apps will have their privacy undermined."
The wide-ranging legislation aims to regulate internet content to keep people safe, and would give media regulator Ofcom the power to demand that platforms identify and remove child abuse content.
Refusing to comply could see companies face huge fines.