U.S. and British spies hacked into the world's biggest maker of phone SIM cards, allowing them to potentially monitor the calls, texts and emails of billions of mobile users around the world, an investigative news website reported. The alleged hack on Gemalto (GTO.AS), if confirmed, would expand the scope of known mass surveillance methods available to U.S. and British spy agencies to include not just email and web traffic, as previously revealed, but also mobile communications. The Franco-Dutch company said on Friday it was investigating whether the U.S. National Security Agency (NSA) and Britain's GCHQ had hacked into its systems to steal encryption keys that could unlock the security settings on billions of mobile phones. The report by The Intercept site, which cites documents provided by former NSA contractor Edward Snowden, could prove an embarrassment for the U.S. and British governments. It opens a fresh front in the dispute between civil liberties campaigners and intelligence services which say their citizens face a grave threat of attack from militant groups like Islamic State. It comes just weeks after a British tribunal ruled that GCHQ had acted unlawfully in accessing data on millions of people in Britain that had been collected by the NSA.