FRANKFURT - US 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, if confirmed, would expand the scope of known mass surveillance methods available to US 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 US 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 US and British governments.

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.

The Intercept report (http://bit.ly/19E0KUK) said the hack was detailed in a secret 2010 GCHQ document and allowed the NSA and GCHQ to monitor a large portion of voice and data mobile communications around the world without permission from governments, telecom companies or users.

"We take this publication very seriously and will devote all resources necessary to fully investigate and understand the scope of such sophisticated techniques," said Gemalto, whose shares sunk by as much as 10 percent in early trading on Friday, following the report.

The report follows revelations from Snowden in 2013 of the NSA's Prism programme which allowed the agency to access email and web data handled by the world's largest Internet companies, including Google, Yahoo and Facebook.

A spokeswoman for Britain's GCHQ (Government Communication Headquarters) said on Friday that it did not comment on intelligence matters. The NSA could not be immediately reached for comment.