The German intelligence "double agent" who allegedly sold hundreds of top secret documents to the Americans was caught by his own country's counter espionage agents while trying to broker an additional spying deal with the Russian secret services, according to intelligence sources in Berlin.

The bizarre twist to Germany's deepening "double agent affair" emerged yesterday after Chancellor Angela Merkel's government summoned the US ambassador to Berlin to "clarify" the second explosive case of suspected American intelligence spying against Germany within the space of a year.

On Friday, Germany's Bundesnachrichtendienst (BND) intelligence service admitted that one of its agents, an as yet unnamed 31-year-old man "with a limp", had been arrested for supplying American intelligence with at least 300 top secret documents over a period lasting several years.

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"The matter is serious," declared a German government spokesman. The disclosure came eight months after German-US relations suffered their worst crisis in decades with the revelation that the US National Security Agency had bugged Chancellor Merkel's mobile phone.

The tapping was carried out from a listening post on the roof of the US embassy in Berlin. GCHQ operates a similar listening post from the roof Britain's embassy in Berlin. German intelligence sources have provided some embarrassing details about the "US mole" who appears to have operated undetected within their ranks for years.

The BND agent, now in German police custody, first established contact with his US spymasters through a simple email he sent to the American embassy which held out the prospect of possible "co-operation". The offer was evidently gladly received. In his role as America's mole within the BND, the agent is thought to have downloaded an estimated 300 secret intelligence documents from the BND's computer system on to USB sticks.

From 2012 until last week, the agent met his US spymasters at secret locations in Austria, where the information was handed over. The agent is said to have received €10,000 (£8,000) per batch of information. The BND has not disclosed what type of material was stolen. However, news of his arrest surfaced during a meeting of a German parliamentary committee which has been investigating the NSA's surveillance and phone-tapping operation.

The double agent's suspected supply of BND information about the workings of the NSA parliamentary committee has raised additional alarm in Germany. "This amounts to an unpardonable attack on parliamentary freedom," protested the Social Democrat parliamentary leader, Thomas Oppermann.

However, the double agent appeared to have been motivated less by ideology than hard cash. German sources revealed yesterday that his activities were only noticed after he tried to widen his information supply portfolio to include the Russians. Three weeks ago BND counter-espionage agents picked up an email that had been sent to the Russian embassy. Like the email to the American embassy, it was said to have offered the possibility of co-operation. "The counter-espionage agents were stunned when they discovered that the source of the email was the BND," reported the German magazine Der Spiegel.

German MPs were yesterday still reeling from the shock of the double agent's unmasking and the alarming realisation that their own domestic intelligence service is not immune to NSA infiltration. "All co-operation between German intelligence and its friendly allies must be thoroughly examined," insisted the Green Party MP Katrin Göring-Eckardt.

There was speculation among some MPs that Germany might "pay back" America by inviting the US whistleblower Edward Snowden to travel from exile in Russia to testify against the NSA before a Berlin parliamentary committee. Chancellor Merkel's government has so far ruled out inviting Mr Snowden for fear of upsetting the White House.