Two of the world's biggest technology companies, Microsoft and Yahoo, expressed deep concern on Friday about widespread attempts by the US and UK intelligence services to circumvent the online security systems that protect the privacy of millions of people online.

Microsoft said it had "significant concerns" about reports that the National Security Agency and its British counterpart, GCHQ, had succeeded in cracking most of the codes that protect the privacy of internet users. Yahoo said it feared "substantial potential for abuse". Google said it was not aware of any covert attempts to compromise its systems.

Documents obtained by whistleblower Edward Snowden and published jointly by the Guardian, the New York Times and the nonprofit 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 (the Microsoft email service now known as Outlook), Google, Yahoo and Facebook.