Investigators in Poland say they have evidence that leaves “no doubt” that Lech Walesa, the dockyard electrician who led the battle to free the country from communism, collaborated with the secret police in the 1970s.

The Institute of National Remembrance, the body charged with investigating communist-era crimes in Poland, said handwriting experts had concluded that Mr Walesa had written or signed a number of secret police documents.

Detailing payments and reports, one of the documents is a handwritten note stating a commitment to collaborate, and signed “Lech Walesa. Bolek:” Bolek being the name believed to be Mr Walesa’s secret police codename.

The documents were discovered a year ago in the house of the late General Czeslaw Kiszczak, the last communist interior minister of Poland.

“The conclusions are clear and leave no doubt,” said Andrzej Pozorski, the Institute of National Remembrance, at a press conference on Tuesday.

“There is a commitment to co-operate and the receipts for money he received,” he continued, adding Mr Walesa worked with secret police from 1970 to 1974.