Privacy International said it had long suspected that members of Five Eyes have been playing "a game of jurisdictional arbitrage to sidestep domestic laws governing interception and collection of data".
"Secret agreements such as these must be placed under the microscope to ensure they are adequately protecting the rights of British citizens," said Eric King, the group's head of research.
"The British government has repeatedly insisted that appropriate warrants were in place in all instances of international intelligence collaboration. We now know this isn't the whole truth. Trust must be restored, and our intelligence agencies must be brought under the rule of law. Transparency around an accountability for these secret agreements is a crucial first step."