The US National Security Agency has secretly broken into the main communications links that connect Yahoo and Google data centres around the world, the Washington Post reports.
The newspaper cites documents obtained from former NSA contractor Edward Snowden and interviews with officials.
According to a secret accounting dated January 9, 2013, NSA sends millions of records every day from Yahoo and Google internal networks to data warehouses at the agency's Fort Meade, Maryland, headquarters.
In the last 30 days, the report on Wednesday's Post website says, field collectors had processed and sent back more than 180 million new records - ranging from "metadata", which would indicate who sent or received emails and when, to content such as text, audio and video.
The NSA's principal tool to exploit the data links is a project called MUSCULAR, operated jointly with the agency's British counterpart, GCHQ.
The Post said NSA and GCHQ are copying entire data flows across fibre-optic cables that carry information between the data centres of the Silicon Valley giants.
White House officials and the Office of the Director of National Intelligence, which oversees the NSA, declined to comment, the Post says.
In a statement to the Post, Google said it was "troubled by allegations of the government intercepting traffic between our data centres, and we are not aware of this activity".
At Yahoo, a spokeswoman said: "We have strict controls in place to protect the security of our data centres, and we have not given access to our data centres to the NSA or to any other government agency."
