Whistleblowing site WikiLeaks on Monday accused Google of handing over the emails and electronic data of its senior staff to the US authorities without providing notification until almost three years later.
Google was apparently acting in response to warrants issued by the US Department of Justice, which is investigating WikiLeaks for publishing hundreds of thousands of classified military and diplomatic files.
WikiLeaks said the allegations against it point to a far broader investigation into its activities than the US authorities have previously indicated.
Alleged offences range from espionage to theft of US government property and computer fraud and abuse, it said.
"Today, WikiLeaks' lawyers have written to Google and the US Department of Justice concerning a serious violation of the privacy and journalistic rights of WikiLeaks' staff," the site said in a statement.
WikiLeaks said that Google could and should have resisted complying with the warrants, as well as immediately informing those whose data it handed over.
The warrants demanded emails, contacts and IP addresses relating to the Google accounts of investigations editor Sarah Harrison, section editor Joseph Farrell and spokesman Kristinn Hrafnsson.
The information was handed over to the US authorities on April 5, 2012, but Google did not inform the WikiLeaks staff until December 23, 2014, according to documents obtained by AFP.
"While WikiLeaks journalists, perhaps uniquely, do not use Google services for internal communications or for communicating with sources, the search warrants nonetheless represent a substantial invasion of their personal privacy and freedom," the organisation added.
WikiLeaks has been targeted by the US authorities since its release in 2010 of 500,000 secret military files on the wars in Afghanistan and Iraq and 250,000 diplomatic cables.
A former army intelligence analyst, Chelsea Manning, is currently serving a 35-year prison term for leaking classified documents to WikiLeaks.
WikiLeaks founder Julian Assange also believes he is a target for prosecution and has been holed up at the Ecuadoran embassy in London since 2012.
He sought asylum there to avoid being sent to Sweden, where he faces allegations of rape and sexual molestation which he denies. He says his extradition to Sweden could see him transferred on to the United States.
In a statement to AFP, Google said it did not comment on individual cases, but said: "Obviously, we follow the law like any other company.
"When we receive a subpoena or court order, we check to see if it meets both the letter and the spirit of the law before complying.
"And if it doesn't we can object or ask that the request is narrowed. We have a track record of advocating on behalf of our users."
It is not the first time WikiLeaks has clashed with the online giant.
In September 2014, Assange published a book, "When Google Met WikiLeaks", questioning the Internet firm's close ties with the US administration.
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