WhatsApp has told the BBC it is supporting fellow tech giant Apple in its legal fight against the UK Home Office over the privacy of its users’ data.

The messaging app’s boss, Will Cathcart, said the case “could set a dangerous precedent” by “emboldening other nations” to seek to break encryption, which is how tech firms keep customers’ information private.

Apple went to the courts after receiving a notice from the Home Office demanding the right to access the data of its global customers if required in the interests of national security.

It and other critics of the government’s position say the request compromises the privacy of millions of users.

The Home Office told the BBC it would not comment on ongoing legal proceedings.

“But more broadly, the UK has a longstanding position of protecting our citizens from the very worst crimes, such as child sex abuse and terrorism, at the same time as protecting people’s privacy,” it said in a statement.

WhatsApp has applied to submit evidence to the court which is hearing Apple’s bid to have the Home Office request overturned.

Mr Cathcart said: “WhatsApp would challenge any law or government request that seeks to weaken the encryption of our services and will continue to stand up for people’s right to a private conversation online.”

This intervention from the Meta-owned platform represents a major escalation in what was an already extremely high-profile and awkward dispute between the UK and the US.

Apple’s row with the UK government erupted in February, when it emerged ministers were seeking the right to be able to access information secured by its Advanced Data Protection (ADP) system.

The argument intensified in the weeks that followed, with Apple first pulling ADP in the UK, and then taking legal action against the Home Office.

It also sparked outrage among US politicians, with some saying it was a “dangerous attack on US cybersecurity” and urging the US government to rethink its intelligence-sharing arrangements with the UK if the notice was not withdrawn.

Tulsi Gabbard, the director of US National Intelligence, described it as an “egregious violation” of US citizens’ privacy.

Civil liberties groups also attacked the UK government, saying what it was demanding had privacy and security implications for people around the world.

The campaign organisation Open Rights Group welcomed WhatsApp seeking to become involved in the case.

“WhatsApp’s intervention shows the breadth of concern about the threat to privacy and security,” said Jim Killock, its executive director.

“It’s important that the court hears from as many companies and organisations as possible so they understand the full impact of what the Home Office is trying to do,” he added.

Apple’s ADP applies end-to-encryption (E2EE) to files such as photos and notes stored on the iCloud, meaning only the user has the “key” required to view them.

The same technology protects a number of messaging services, including WhatsApp.

That makes them very secure but poses a problem for law enforcement agencies.

They can ask to see data with lower levels of protection – if they have a court warrant – but tech firms currently have no way to provide access to E2EE files, because no such mechanism currently exists.

Tech companies have traditionally resisted creating such a mechanism not just because they say it would compromise users’ privacy but because there would be no way of preventing it eventually being exploited by criminals.

In 2023, WhatsApp said it would rather be blocked as a service than weaken E2EE.

When Apple pulled ADP in the UK it said it did not want to create a “backdoor” that “bad actors” could take advantage of.

Further complicating the argument is that the Home Office has submitted its request to Apple via what it is known as a Technical Capability Notice (TCN), something which by law is secret

Neither Apple nor the Home Office has confirmed its existence. WhatsApp says so far it has not received a TCN.

When the matter came to court, government lawyers argued that the case should not be made in public in any way for national security reasons.

However, in April, a judge agreed with a number of news organisations, including the BBC, and said certain details should be made public.

“It would have been a truly extraordinary step to conduct a hearing entirely in secret without any public revelation of the fact that a hearing was taking place,” his ruling stated.

In its statement to the BBC, the Home Office said: “The UK has robust safeguards and independent oversight to protect privacy and these specific powers are only used on an exceptional basis, in relation to the most serious crimes and only when it is necessary and proportionate to do so.”