Apple, eBay and Amazon have joined forces with Microsoft in the software company's legal battle with the US government over access to customer data stored in Ireland.
A diverse set of interests have filed briefs with a federal appeals court in New York, urging it to reverse a judge's order that Microsoft turn over emails from a Dublin data centre.
They argued that turning them over would jeopardise the future of international cloud computing.
The case has taken on urgency for tech and media companies in the wake of revelations about bulk electronic data collection by the US National Security Agency from former NSA contractor Edward Snowden.
Technology companies including Cisco, HP and rival Apple filed briefs alongside media companies, academics and lobby groups to support Microsoft's bid to fend off a government search warrant for the emails.
Microsoft began fighting the warrant in 2013, saying that US prosecutors were overreaching by demanding data held in a foreign country without the assistance of local authorities.
It is not known whose emails are sought, but prosecutors said they wanted them for a drug investigation.
The prosecutors said their demand did not violate Irish sovereignty because Microsoft's US employees had control of the emails and could retrieve them without going to Ireland.
In July, US District Judge Loretta Preska agreed and ordered Microsoft to comply.
The ruling could wreak havoc in cloud computing for both businesses and individuals if countries regularly begin to claim authority over data stored elsewhere, said Andrew Pincus, a lawyer who filed a brief for groups including the US Chamber of Commerce, the largest US business lobby.
Businesses want to be sure their legal records, intellectual property and merger plans are generally private from authorities worldwide, Pincus said.
"If by putting them in the cloud, you lose control over them and the government just gets access whenever it wants, nobody's going to do that," Pincus said at a conference at Microsoft's New York office.
The security of data centres is critical to journalists, said Bruce Brown, executive director of the US-based Reporters Committee for Freedom of the Press.
"We have stuff governments around the world want," Brown said at the conference.
Others supporting Microsoft in court briefs included the American Civil Liberties Union and 35 computer science and engineering professors.
A ruling by the appeals court is likely months away. A Justice Department spokesman declined to comment.