Google has enhanced the encryption technology for its flagship e-mail service, Gmail - used by more than 425 million people - so that it will be more difficult for the US National Security Agency to intercept messages moving among the company's worldwide data centres.
Among the most extraordinary disclosures in documents leaked by former NSA systems analyst Edward Snowden were reports that the NSA had secretly tapped into the main communications links that connect Yahoo and Google data centres.
Eric Schmidt, Google's executive chairman, who in November said he was outraged over the practice, did not mention the NSA in Thursday's announcement, except in a veiled reference to "last summer's revelations".
Yahoo promised similar steps to its e-mail service this spring.
Technology companies like Google give information to the NSA and government agencies when required by a court order.
"Google is making it tougher for the government to spy on its customers without going through Google," said Chris Soghoian, a senior policy analyst at the American Civil Liberties Union. "There are still ways for the NSA to spy on the bad guys. But this will prevent them spying on 500 million people at once."
Google and other technology companies have been outspoken about the US government's spy programmes. The companies are worried more people will reduce their online activities if they believe almost everything they do is being monitored by the government. A drop in internet use could hurt the companies financially.
"Your e-mail is important to you, and making sure it stays safe and always available is important to us," Nicolas Lidzborski, Gmail's security engineering lead, wrote in a blog post.
He said all Gmail messages consumers sent or received were now encrypted. "This ensures your messages are safe, not only when they move between you and Gmail's servers, but also as they move between Google's data centres - something we made a top priority after last summer's revelations."
The NSA said it focusesd only on targets with foreign intelligence value.
Documents released by Snowden last year claimed a secret January 9, 2013 accounting showed the NSA sends millions of records every day from Yahoo and Google internal networks to its Maryland data warehouses. The NSA and its British counterpart, GCHQ, exploit Google and Yahoo data using a project called MUSCULAR.
They copy entire data flows across fibre-optic cables that carry information between the data centres of the Silicon Valley giants, the Washington P ost said.