The two-page memorandum challenges that official line, carrying an acknowledgement by Britain's electronic eavesdropping agency GCHQ that it scoops up an undisclosed amount of "unanalyzed intercepted communications content" from its foreign intelligence partners without a warrant.
For human rights groups challenging U.K. surveillance, the document shows that GCHQ can use its allies to engage in mass spying without proper oversight.
"The British public had been consistently persuaded by government over the last year that GCHQ are not using the NSA as a backdoor to sidestep protection in the law," said Eric King, whose group, Privacy International, has joined London-based rights group Liberty and Amnesty International in suing the spy agency.
King, whose group obtained the memo as part of its suit, said it shows "a huge gap in protection for British people."
GCHQ declined to comment.
Andrew Defty, who teaches politics at the University of Lincoln, said GCHQ was probably right to argue that the exchange of unfiltered raw intelligence isn't restricted by Britain's intercept legislation, known as RIPA.
"This is in all likelihood not illegal," said Defty, the co-author of "Watching the Watchers," a book about parliamentary oversight of Britain's intelligence services. "However, it does appear to go against the spirit of the law, which would seem to rest on the principle that interception must take place for a reason."
Defty said the revelation illustrates the need to update Britain's intercept law after the revelations by former NSA contractor Edward Snowden about mass spying.
"RIPA needs replacing," he said.