News reports suggest US and UK British Intelligence agencies illegally hacked into a major manufacturer of Sim cards to steal codes and facilitate eavesdropping on mobiles.
The Intercept website said that the information came from former American intelligence contractor Edward Snowden.
Gemalto - the company allegedly targeted - says it is taking the allegations "very seriously". The company makes Sim cards for mobile phones and furnishes service providers with encryption codes to keep the data on each phone private.
It operates in 85 countries and has more than 40 manufacturing facilities.
The Intercept says that the "great Sim heist" gave US and British surveillance agencies "the potential to secretly monitor a large portion of the world's cellular communications, including both voice and data."
It says that among the clients of the Netherlands-based company are AT&T, T-Mobile, Verizon, Sprint and some "450 wireless network providers around the world".
It also claims that the hack, organised by Britain's GCHQ and America's National Security Agency, took place in 2010, with neither agency commenting on the allegations.
The stolen encryption allows agencies to decode data that passes between mobile phones and cell towers.
They were able to grab calls, texts or emails intercepted out of the air.
A spokeswoman for Gemalto said that while the company was not targeted there was "an attempt to try and cast the widest net possible, to reach as many mobile phones as possible."
She added: "We take this publication very seriously and will devote all resources necessary to fully investigate and understand the scope of such highly sophisticated techniques to try to obtain Sim card data."
Correspondents say that the revelations are highly embarrassing for the agencies because they give the impression that they will do whatever is required to improve their surveillance powers, even if that means stealing data from law-abiding Western firms.
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