The man, who is German, has admitted passing to an American contact details about a special German parliamentary committee set up to investigate the spying revelations made by former U.S. intelligence contractor Edward Snowden, the politicians said.
Both lawmakers are members of the nine-person parliamentary control committee, whose meetings are confidential, and which is in charge of monitoring the work of German intelligence agencies.
The parliamentary committee investigating the NSA affair also holds some confidential meetings.
The German Foreign Ministry said in a statement that it had invited the U.S. ambassador to come for talks regarding the matter, and asked him to help deliver a swift explanation.
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"This was a man who had no direct contact with the investigative committee ... He was not a top agent," said one of the members of parliament, who spoke on condition of anonymity. The suspect had offered his services to the United States voluntarily, the source said.
Merkel's spokesman Steffen Seibert said: "We don't take the matter of spying for foreign intelligence agencies lightly."
When asked whether Merkel had discussed the issue with President Barack Obama during a phone conversation on Thursday night, he merely said they had talked about foreign affairs.
The U.S. embassy in Berlin, the State Department in Washington and the White House all declined to comment.
Germany is particularly sensitive about surveillance because of abuses by the Stasi secret police in communist East Germany and by the Nazis. After the Snowden revelations, Berlin demanded that Washington agree to a "no-spy agreement" with its close ally, but the United States has been unwilling.
Germany's Sueddeutsche Zeitung newspaper and the broadcasters WDR and NDR reported that the alleged spy was first detained on suspicion of contacting Russian intelligence agents. He then admitted he had worked with Americans.
Bild newspaper said in an advance copy of an article to be published on Saturday that the man had worked for two years as a double agent and had stolen 218 confidential documents.
He sold the documents, three of which related to the work of the committee in the Bundestag, for 25,000 euros ($34,100), Bild said, citing security sources.
Opposition lawmakers called for diplomatic consequences if the allegations should prove true.
The head of parliament's committee investigating the NSA affair, Patrick Sensburg, said its members had long feared they might be targeted by foreign intelligence agents and had taken special measures.
($1 = 0.7331 Euros)
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