A broad array of technology firms and activists joined Apple's legal fight over encryption Thursday, warning of a dangerous precedent if the company is forced to help the government break into a locked iPhone.

Three tech associations representing Apple's main business rivals -- including Google, Facebook, Microsoft and Yahoo -- announced a joint brief supporting Apple's efforts to challenge an order that would require it to help unlock an iPhone used by one of the San Bernardino attackers.

"If the government arguments prevail, the Internet ecosystem will be weakened, leaving Internet users more vulnerable to hackers and other bad actors," said a statement from the Computer and Communications Industry Association (CCIA), which announced a joint amicus brief with the Internet Association and the i2Coalition of Internet infrastructure firms.

The three associations said their brief was set to be filed before the midnight deadline in federal court in California, where the case is being heard.

A number of other companies and associations are also expected to file briefs in the case, which has divided the American public and set off a highly charged debate about what limits should be placed on law enforcement access to digital devices.

"There is broad and deep concern throughout many types of companies throughout the tech industry that there is a potentially dangerous precedent in this case," said Ed Black, president and chief executive of CCIA.

A joint filing was being prepared by Google, Facebook, Microsoft, Snapchat, Evernote and Mozilla, according to Denelle Dixon-Thayer, chief legal officer for Mozilla Corporation.

"Tech companies should aspire to build 'unhackable' products," Dixon-Thayer said in a blog post.

"With this precedent, we could all be told not to build secure products in the first place."

The case stems from the FBI's efforts to access the locked iPhone used by one of the perpetrators of the December attack in San Bernardino, California that killed 14 people.

Apple has argued that the only way to unlock the phone would be to introduce a weakened operating system, which could potentially leak out and be exploited by hackers and foreign governments.

The FBI has argued that by introducing encryption that can lock data, making it accessible only to the user, Apple and others are essentially creating "warrant-proof zones" for criminals and others that will cripple law enforcement and jeopardize public safety.

The CCIA includes Apple rivals such as Amazon, Pandora and Samsung, and the Internet Association counts as members Dropbox, Microsoft, Yahoo and Facebook. Some firms are members of both, such as Google, which is also part of the i2Coalition.

Protesters demonstrate outside an Apple Store as they object to the US Government's attempt to put a backdoor to hack into the Apple iPhone, in Los Angeles, California on February 23, 2016. Photo: AFP

Twitter, cryptographers for Apple

Twitter meanwhile filed another legal brief supporting Apple, together with 16 other tech firms including eBay, LinkedIn, Airbnb and Reddit.

"The government seeks unbounded authority to compel Apple to design software that does not currently exist and that will circumvent and undermine security measures intended to protect its users' data," the brief said.

This could "set a precedent that could be used in future cases to require (other tech firms) to provide technical assistance in a manner that undermines the very products they offer."

Intel announced its own legal filing supporting Apple, saying that it would be "an unprecedented step for the government to require a company to develop technology that weakens security in a commercial product."

"Such a move chills innovation," the chipmaker said in a statement.

AT&T joined the effort as well, saying the order expands law enforcement authority.

"Only Congress can address these issues in a sufficiently comprehensive, uniform, and fair manner," AT&T said.

A group of cryptographic experts also filed arguments in support of Apple's case on Thursday.

Forcing Apple to create a new operating system "increases the risk that the forensic software will escape Apple's control either through theft, embezzlement, or order of another court, including a foreign government," said the brief filed by the Stanford Law School Center for Internet and Society on behalf of several academic and private sector cryptographers.

Digital rights groups also backed Apple, with the Electronic Frontier Foundation arguing in its brief that the order would violate Apple's free speech rights by forcing it to write weakened software guaranteed with its digital signature.

It would be "akin to the government dictating a letter endorsing its preferred position and forcing Apple to transcribe it and sign its unique and forgery-proof name at the bottom," EFF argued in its brief endorsed by 46 technology specialists.

A separate filing by the Center for Democracy & Technology said the order "would set a precedent under which any company could be forced to spy on unknowing customers on behalf of law enforcement, and in the process be required to override its own security measures in ways that expose its users to malicious attacks."