Angela Merkel REUTERS/Michaela Rehle Chancellor Angela Merkel's popularity has dropped abruptly in the wake of an unfolding spying scandal and she has slipped from the top spot among Germany's leading politicians, ARD television network said on Friday.

Merkel, under fire over reports the BND intelligence agency helped the U.S. National Security Agency (NSA) spy on European targets with her office's blessings, saw her approval rating fall five points to 70 percent in the Infratest Dimap poll.

Although she remains very popular in Germany, that was an unusually steep one-month drop in a survey that normally moves by just a point or two each month, and it was the lowest level for Merkel since her 65-percent approval rating in December.

Foreign Minister Frank-Walter Steinmeier, a leading Social Democrat (SPD) politician untouched by the spying scandal that has gripped the country, moved ahead of Merkel into the top spot in the poll with a 74 percent approval rating.

SPD chairman Sigmar Gabriel, who pointedly raised questions in public about Merkel's role in the affair that then put strains on the grand coalition government, saw his approval rating jump five points to 53 points.

Merkel's Christian Democrats govern in a grand coalition with the center-left SPD.

The approval rating for Interior Minister Thomas de Maiziere, a former Merkel chief of staff now under fire for his role in the spying scandal, plunged a steep 15 points to 38 points, ARD said.

The poll of 1,000 voters found 70 percent believe Berlin's control of its intelligence agencies is insufficient.

The chancellery has said it knew of NSA interest in spying on European defense firms since 2008, even though parliament was told in 2014 it had no information on that.

A separate INSA poll on Wednesday showed that 62 percent of Germans think the BND row threatens Merkel's credibility.

In office since 2005, Merkel - dubbed "Mutti" (Mommy) by the media for the way she reassures many German voters - has long been the country's most popular politician and has weathered several earlier controversies.

But Germans are upset that the BND may have helped the NSA spy on European officials and firms such as Airbus. Spying for the NSA is a sensitive issue in Germany due to past abuses of privacy rights by the Nazis and East German Stasi secret police.

Merkel ardently backed the BND's cooperation on Monday, saying it was vital to fight terrorism.