US authorities have reportedly investigated claims that Meta can read users’ encrypted chats on the WhatsApp messaging platform, which it owns.

The reports follow a lawsuit filed last week, which claimed Meta “can access virtually all of WhatsApp users’ purportedly ‘private’ communications”.

Meta has denied the allegation, reported by Bloomberg, calling the lawsuit’s claim “categorically false and absurd”. It suggested the claim was a tactic to support the NSO Group, an Israeli firm that develops spyware used against activists and journalists, and which recently lost a lawsuit brought by WhatsApp.

The firm that filed last week’s lawsuit against Meta, Quinn Emanuel Urquhart & Sullivan, attributes the allegation to unnamed “courageous” whistleblowers from Australia, Brazil, India, Mexico and South Africa.

Quinn Emanuel is, in a separate case, helping to represent the NSO Group in its appeal against a judgment from a US federal court last year, which ordered it to pay $167m to WhatsApp for violating its terms of service in its deployment of Pegasus spyware against more than 1,400 users.

“We’re pursuing sanctions against Quinn Emanuel for filing a meritless lawsuit that was designed purely to grab headlines,” said Carl Woog, a Meta spokesperson, in a statement. “This is the same firm that is trying to help NSO overturn an injunction that barred their operations for targeting journalists and government officials with spyware.”

Adam Wolfson, a partner at Quinn Emanuel said: “Our colleagues’ defence of NSO on appeal has nothing to do with the facts disclosed to us and which form the basis of the lawsuit we brought for worldwide WhatsApp users.

“We look forward to moving forward with those claims and note WhatsApp’s denials have all been carefully worded in a way that stops short of denying the central allegation in the complaint – that Meta has the ability to read WhatsApp messages, regardless of its claims about end-to-end encryption.”

Steven Murdoch, professor of security engineering at UCL, said the lawsuit was “a bit strange”. “It seems to be going mostly on whistleblowers, and we don’t know much about them or their credibility,” he said. “I would be very surprised if what they are claiming is actually true.”

If WhatsApp were, indeed, reading users’ messages, this was likely to have been discovered by staff and would end the business, he said. “It’s very hard to keep secrets inside a company. If there was something as scandalous as this going on, I think it’s very likely that it would have leaked out from someone within WhatsApp.”

The Bloomberg article cites reports and interviews from officials within the US Department of Commerce in claiming that the US has investigated whether Meta could read WhatsApp messages. However, a spokesperson for the department called these assertions “unsubstantiated”.

WhatsApp bills itself as an end-to-end encrypted platform, which means that messages can be read only by their sender and recipient, and are not decoded by a server in the middle.

This contrasts with some other messaging apps, such as Telegram, which encrypt messages between a sender and its own servers, preventing third parties from reading the messages, but allowing them – in theory – to be decoded and read by Telegram itself.

A senior executive in the technology sector told the Guardian that WhatsApp’s vaunted privacy “leaves much to be desired”, given the platform’s willingness to collect metadata on its users, such as their profile information, their contact lists, and who they speak to and when.

However, the “idea that WhatsApp can selectively and retroactively access the content of [end-to-end encrypted] individual chats is a mathematical impossibility”, he said.

Woog, of Meta, said: “We’re pursuing sanctions against Quinn Emanuel for filing a meritless lawsuit that was designed purely to grab headlines. WhatsApp’s encryption remains secure and we’ll continue to stand up against those trying to deny people’s right to private communication.”