Vladimir Putin
Russian President Vladimir Putin in China for energy talks.

Russian Prime Minister Vladimir Putin has been labeled as a wife-beater and adulterer by Germany's spy agency BND, according to secret files discovered by Erich Schmidt-Eenboom, expert and author of several books on the organisation. The files, published in the German media, also nourish the rumour that Putin might have left an illegitimate baby behind, Germany's Bild newspaper has claimed.

Putin, 59, fell victim of BND's sting when his wife Lyudmila became close friend with Lenchen, a Baltic-German female agent posed as interpreter at the KGB office. She quickly opened her heart about her husband's alleged infidelities and abuse to the German spy in disguise, known in the BND as "Balcony" because of her large breasts.

"Contrary to the picture she would paint later of an ideal marriage, [Lyudmila] complained to her confidante about domestic violence and numerous affairs," Mr Schmidt-Eenboom claimed in a paper on the BND's Dresden spy operation. "These reports were shared with Nato allies."

Officially, Putin was captain in Dresden for the "Group of Soviet Forces in Germany" from 1985 onwards. In fact, he worked for the KGB's foreign intelligence service and recruited agents. In the spring of 1990 he left Dresden to Leningrad.

Mrs Putin has always referred to the German years as the happiest of her marriage with the Russian Prime Minister, thanks also to the birth of their two daughters in 1985 and 1986. She is very rarely seen in public with her husband, feeding rumours about the state of the marriage.

In 2008, it allegedly emerged that Putin had secretly break up with his wife and was about to wed Olympic champion gymnast Alina Kabayeva. The Prime Minister immediately denied any allegation.

The German spy has been given a new identity and settled in the south of the country. Dieter Arndt, a spokesman for the BND, said: "We do not discuss our methods or our investigations."

Putin, who is the favourite in the next year's presidential election, was recently listed by Forbes magazine as the second most powerful man in the world after US President Barack Obama.