Two years ago, Germany complained bitterly when whistleblower Edward Snowden revealed the extent of U.S. spying in Europe, which reportedly included monitoring Chancellor Angela Merkel's cellphone.

Now there is a new scandal infuriating Germans. Only this time, the outrage is not because the country was a victim of aggressive intelligence gathering, but because reports suggest it was a willing participant.

Germany's foreign intelligence service, the BND, allegedly carried out extensive surveillance at the request of the U.S. National Security Agency. The targets reportedly included European companies, institutions and individuals, violating German policy and possibly breaking the law.

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The affair raises pointed questions about what the highest levels of the German government knew about the alleged spying, when they knew it and how they responded. It has rapidly turned into a major challenge for Ms. Merkel, whose rivals have seized the chance to dent her considerable popularity.

In a sign of the burgeoning controversy, the BND stopped fulfilling U.S. requests for Internet-related surveillance, German news outlets reported on Thursday. Such requests made up the vast majority of those forwarded to a BND listening station in Bavaria, the reports said.

German lawmakers are calling for a full list of the questionable targets – e-mail addresses, Internet protocol addresses and telephone numbers – to be turned over to a parliamentary committee. The committee questioned Thomas de Maizière, the Interior Minister, behind closed doors on Wednesday. Ms. Merkel has also said she would testify if needed.

The spying controversy is already tarnishing Ms. Merkel's reputation for competence and credibility. A poll conducted this week found that 62 per cent of those surveyed said the situation had put her trustworthiness at risk. A German tabloid, meanwhile, earlier printed a photo of Mr. de Maizière with his nose lengthened in a parody of Pinocchio.

Then there's the additional element of perceived hypocrisy. Back in 2013, Ms. Merkel memorably said that "spying among friends is not acceptable." Now, by carrying out the U.S. requests, Germany is being accused of doing what Ms. Merkel deplored.

Earlier this week, Austria said it had filed a legal complaint in connection with reports that its government agencies were among the targets of the spying. Airbus Group, the Franco-German aerospace giant, has also registered a complaint that it was targeted for industrial espionage.

The latest developments are also stirring tensions within Germany's coalition government. Ms. Merkel's centre-right Christian Democrats govern with the help of a junior partner, the centre-left Social Democrats. While the two parties are currently allies, they are also rivals, particularly as the next federal election in 2017 draws nearer.

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The spying affair provides, for the first time, an "opportunity to attack the chancellor directly," said Jens Walther, a researcher at Dusseldorf University who studies German politics. As a result, "the esteem for Ms. Merkel and her high reputation among Germans could sink."

The uproar in Germany is another chapter in the ongoing debate over how to balance the need for surveillance with the protection of individual rights. France recently adopted legislation granting sweeping new powers to its security and intelligence apparatus. Canada also just expanded the tools available to its law-enforcement authorities and spy agency in the fight against violent extremists.

Germans have an aversion to government violations of privacy, a legacy of the country's experience with surveillance both by the Nazi regime and the East German communists. The revelation that the NSA was collecting vast amounts of metadata was anathema here. (The data collection remains controversial in the U.S. too: on Thursday, a federal appeals court in New York ruled it was not authorized under U.S. law.)

The U.S. and Germany have a long history of co-operation in intelligence matters, a collaboration which deepened after the attacks of Sept. 11, 2001. In remarks earlier this week, Ms. Merkel acknowledged there was an "innate tension" between security and privacy. At the same time, she said, the BND "must and will co-operate internationally to protect the bodies and lives of 80 million Germans as best they can" – first and foremost with the NSA.

The question raised by the current scandal is whether during the course of that co-operation the NSA began to expand the scope of its requests to the German intelligence service into areas well beyond anti-terrorism activities. Such requests – for example, targeting French diplomats rather than suspected extremists – may have violated the terms of the agreement governing the intelligence sharing between the U.S. and Germany.

Another critical issue is whether the BND and the Chancellor's office knew of the alleged improper requests and failed to prevent them. The German parliamentary committee that has been investigating the NSA's activities has heard testimony from former employees of the U.S. agency; one of them asserted last year that the BND operated like an "appendage of the NSA," according to newspaper reports.

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Ms. Merkel "must now personally answer these allegations and explain the unclear role of the chancellery in supervising the BND," said Konstantin von Notz, a Greens member of parliament who sits on the committee, in a statement this week.