Rights groups around the world have called for urgent action to protect private communications after it was revealed that US and British spies hacked into the world's largest sim card manufacturer and gained unfettered access to billions of mobile phones around the globe.

The National Security Agency (NSA) and its British equivalent, GCHQ, hacked into Gemalto, a Netherlands sim card manufacturer, stealing encryption keys that allowed them to secretly monitor voice calls and data, according to documents newly released by NSA whistleblower Edward Snowden.

The breach, revealed in documents provided to the Intercept, gave the agencies the power to secretly monitor a large portion of the world's cellular communications, which experts said violated international laws.

Rachel Logan, Amnesty UK's legal director, said: "This mass sim hacking allegation seems be just the latest disturbing revelation about how GCHQ has overreached. These spooks must stop pretending the law doesn't apply to them.

"We keep seeing the intelligence agencies claiming everything's fine and then being caught out when challenged in court."

Mark Rumold, staff attorney at the Electronic Frontier Foundation, said there was no doubt that the spy agencies had violated Dutch law and were in all probability violating laws in many other territories when they used the hacked keys.