Government attempts to circumvent online security systems offer “substantial potential for abuse”, Yahoo has said, in the first response by a tech giant to reports that British and American intelligence services had succeeded in cracking most of the codes that protect the privacy of internet users.

Yahoo said it was unaware of attempts by the National Security Agency (NSA) and its British counterpart, GCHQ, to get around the encryption systems used by the biggest internet companies.

Documents, obtained by whistleblower Edward Snowden and published by The Guardian , The New York Times and the non-profit news organisation ProPublica on Thursday, show that agents at GCHQ have been working to undermine encrypted traffic on the “big four” service providers, named as Hotmail (part of Microsoft), Google, Yahoo and Facebook.

Yahoo responded with a strongly worded statement. “We are unaware of and do not participate in such an effort, and if it exists, it offers substantial potential for abuse. Yahoo zealously defends our users’ privacy and responds to government requests for data only after considering every applicable objection and in accordance with the law,” a spokesman said.

Tensions between tech firms and the U.S. authorities have been escalating. On Monday, Microsoft and Google will file their latest legal briefs in a joint attempt to force the Foreign Intelligence Surveillance Court to allow them to disclose more information about the requests for confidential information they receive.

A spokesman for Google said: “We do not provide any government, including the U.S. government, with access to our systems. As for recent reports that the U.S. government has found ways to circumvent our security systems, we have no evidence of any such thing ever occurring.” Microsoft and Facebook were not immediately available for comment.

— © Guardian Newspapers Limited, 2013