(Reuters) – Alphabet Inc’s Google, Facebook Inc, Microsoft Corp and several other Internet and technology companies will file a joint legal brief on Thursday asking a judge to support Apple Inc in its encryption battle with the U.S. government, sources familiar with the companies’ plans said.

The effort is a rare display of unity and support for the iPhone maker from companies which are competitors in many areas, and shows the breadth of Silicon Valley’s opposition to the government’s anti-encryption effort.

The fight between Apple and the government became public last month when the U.S. Federal Bureau of Investigation obtained a court order requiring Apple to write new software and take other measures to disable passcode protection and allow access to an iPhone used by one of the San Bernardino shooters in December.

Apple has pushed back, arguing that such a move would set a dangerous precedent and threaten customer security. The clash has intensified a long-running debate over how much law enforcement and intelligence officials should be able to monitor digital communications.

The group of tech companies plans to file what is known as an amicus brief – a form of comment from outside groups common in complex cases – to the Riverside, California, federal judge Sheri Pym. She will rule on Apple’s appeal of a court order that would force it to create software to unlock the iPhone.

The companies will contest government arguments that the All Writs Act, a broad 1789 law that enables judges to require actions necessary to enforce their own orders, compels Apple to comply with its request.

In their joint brief, the tech companies will say that Congress passed the All Writs Act before the invention of the light bulb, and that it goes too far to contend that the law can be used to force engineers to disable security protections, according to a source familiar with their arguments.

Google, Facebook and others also appear to be tailoring their arguments specifically to a U.S. Supreme Court audience, where the case may end up. The brief will highlight a unanimous 2014 U.S. Supreme Court case which said law enforcement needs warrants to access smartphones snared in an arrest, the source said.

That opinion, penned by Chief Justice John Roberts, united the Supreme Court’s liberal and conservative factions.

Briefs are also expected in support of the government.

Stephen Larson, a former federal judge, told Reuters last week that he is working on a brief with victims of the San Bernardino shooting who want the FBI to be able to access the data on the phone used by Rizwan Farook. “They were targeted by terrorists, and they need to know why, how this could happen,” Larson said.

Several other tech companies are joining Google, Facebook and Microsoft.

Mozilla, maker of the Firefox web browser, said it was participating, along with online planning tool maker Evernote and messaging app firms Snapchat and WhatsApp. Bookmarking and social media site Pinterest and online storage firm Dropbox are also participating.

“We stand against the use of broad authorities to undermine the security of a company’s products,” Dropbox General Counsel Ramsey Homsany said in a statement.

A separate group including Twitter Inc, eBay Inc, LinkedIn Corp and more than a dozen other tech firms filed a brief with the court in support of Apple on Thursday. AT&T Inc filed its own brief.

Networking leader Cisco Systems Inc said it expected to address the court on Apple’s behalf, but did not say whether it was joining with the large group of companies.

Semiconductor maker Intel Corp plans to file a brief of its own in support of Apple, said Chris Young, senior vice president and general manager for Intel Security Group.

“We believe that tech companies need to have the ability to build and design their products as needed, and that means that we can’t have the government mandating how we build and design our products,” Young said in an interview.

The Stanford Law School for Internet and Society filed a separate brief on Thursday morning on behalf of a group of well-known experts on iPhone security and encryption, including Charlie Miller, Dino Dai Zovi, Bruce Schneier and Jonathan Zdziarski.

Privacy advocacy groups the American Civil Liberties Union, Access Now and the Wickr Foundation filed briefs on Wednesday in support of Apple before Thursday’s deadline set by Pym.

Salihin Kondoker, whose wife Anies Kondoker was injured in the San Bernardino attack, also wrote on Apple’s behalf, saying he shared the company’s fear that the software the government wants Apple to create to unlock the phone could be used to break into millions of other phones.

“I believe privacy is important and Apple should stay firm in their decision,” the letter said. “Neither I, nor my wife, want to raise our children in a world where privacy is the tradeoff for security.”

Law enforcement officials have said that Farook and his wife, Tashfeen Malik, were inspired by Islamist militants when they shot and killed 14 people and wounded 22 others last Dec. 2 at a holiday party. Farook and Malik were later killed in a shootout with police and the FBI said it wants to read the data on Farook’s phone to investigate any links with militant groups.

Earlier this week, a Brooklyn judge ruled that the government had overstepped its authority by seeking similar assistance from Apple in a drug case.

(Reporting by Jim Finkle in Boston and Dustin Volz in San Francisco; Additional reporting by Dan Levine, Heather Somerville, Sarah McBride, Julia Love in San Francisco; Editing by Jonathan Weber, Grant McCool and Bill Rigby)