Germany has directly challenged British ministers over GCHQ's reported programme of the mass monitoring of global phone and internet traffic.
Justice minister Sabine Leutheusser-Schnarrenberger has to written Justice Secretary Chris Grayling and Home Secretary Theresa May questioning the legal basis for the programme code-named Project Tempora, The Guardian reported.
She warned the UK ministers that she intends to raise the issue at next month's meeting of EU justice and home affairs ministers in Brussels.
Her concerns were said to have been reinforced by a phone call from the justice ministry in Berlin to Ursula Brennan, permanent secretary at the Ministry of Justice.
The move reflects growing anger in Germany at the disclosures of the former US intelligence contractor Edward Snowden over the surveillance activities of GCHQ and its American counterpart, the National Security Agency.
In her letter, Ms Leutheusser-Schnarrenberger asks for clarification of the legal basis for Project Tempora and whether it has been authorised by any judicial authority.
She also questions whether the collection of data - which is held for up to 30 days - is triggered by "concrete suspicions" or whether is part of a general trawl.
"I feel that these issues must be raised in a European Union context at minster's level and should be discussed in the context of ongoing discussions on the EU data protection regulation," she wrote.
The Ministry of Justice confirmed last night that it had received Ms Leutheusser-Schnarrenberger's letter and would respond "in due course". A Home Office spokesman said: "We do not routinely comment on private correspondence."
Meanwhile, Foreign Secretary William Hague has declared that Britain should have "nothing but pride" in its "indispensable" intelligence-sharing relationship with the US.
In a speech at the Ronald Reagan Library in California during his visit to the US, he said both nations operated "within a strong legal framework... under the rule of law" and used the information to protect the freedom of their citizens.
"We should have nothing but pride in the unique and indispensable intelligence-sharing relationship between Britain and the United States," he said.
"In recent weeks this has been a subject of some discussion.
"Let us be clear about it: in both our countries intelligence work takes place within a strong legal framework.
"We operate under the rule of law and are accountable for it. In some countries secret intelligence is used to control their people - in ours, it only exists to protect their freedoms."
He noted that democratic societies were now "a world of almost unlimited access to information"- but that the need for libraries would remain.
"This library testifies that it is not enough to believe in our values, we have to defend them and be a beacon of them - all the more so in periods where these values are threatened," he went on.
"Not all countries are willing to exert themselves to defend the freedoms they enjoy, but in the United Kingdom and United States of America we are."
There was "no greater bastion of freedom than the Transatlantic Alliance", he said.