Amnesty International has urged the UK Government to hold an independent judge-led inquiry into surveillance of human rights organisations by the national security services.
In a letter to the British Prime Minister David Cameron, Amnesty demanded an explanation as to why the country has been intercepting, accessing, and storing its communications.
On 1 July, the UK's Investigatory Powers Tribunal (IPT) notified Amnesty that it was subjected to unlawful mass surveillance by the Government Communications Headquarters. IPT oversees surveillance matters in the UK.
Amnesty International secretary-general Salil Shetty said: "The Prime Minister needs to explain why the UK Government is subjecting law-abiding human rights organisations to surveillance.
"This revelation makes it vividly clear that mass surveillance has gone too far. We must finally have proper checks and balances."
Amnesty UK director Kate Allen said: "It's absolutely shocking that Amnesty International's private correspondence was deemed fair game to UK spooks, who have clearly lost all sense of what is proportionate or appropriate.
"A key measure of a free society is how it treats its charities and NGOs. If Amnesty International is being spied on, then is anyone safe?"
The letter warns that confirmation that the country has been spying on the communications of non-governmental organisations 'sends a chilling message to human rights organisations and charities in the UK and abroad.'
In addition, the human rights group is calling on Cameron to publish as much as possible of the findings of a report from the tribunal, which is scheduled to be sent to him relating to the revelation.
In addition to Amnesty, ten NGOs, including the Egyptian Initiative for Personal Rights and South Africa-based Legal Resources Centre, have already launched a legal challenge against suspected unlawful mass surveillance of their work by British spy agencies.
Image: Amnesty warned that the spying sends a chilling message to human rights organisations and charities in the UK and abroad. Photo: Copyright Matt Cardy / Getty Images.