Lech Walesa led the Soviet bloc's first independent trade union, Solidarity
Polish anti-communist leader Lech Walesa has threatened to leave Poland after a second book accused him of being a communist spy as a young man.
The former president and Solidarity leader said he was tired of defending himself against claims he collaborated with the secret police in the 1970s.
He also demanded greater support from the democratic institutions of the state he fought for in the 1980s.
Mr Walesa was cleared of earlier spying allegations by a special court in 2000.
Judges concluded that former SB security service agents had forged documents in his file in a bid to prevent him receiving the Nobel Peace prize in 1983.
Mr Walesa led the Soviet bloc's first independent trade union, Solidarity, which emerged to challenge Poland's communist government during strikes in the Gdansk shipyards in 1980, and eventually helped overthrow it nine years later.
In 1990, he was swept to power as the country's first post-communist president.
The latest book on Mr Walesa's life, which was published earlier this month, repeated a claim that he spied on his colleagues in the Gdansk shipyards in the 1970s. It also alleges he fathered an illegitimate child.
It was written by a 24-year-old historian working at Poland's Institute for National Remembrance (IPN), which investigates communist-era crimes.
On Monday, Mr Walesa threatened to leave Poland and hand back his Nobel Prize if the law and the courts failed to protect him from the "unpunished attacks" on his life story.
"Fascists and communists killed but they murdered their enemies," he said. "Here, friends and patriots are being murdered, and for what price? A historian must decide whether this serves Poland, and not just repeat unlikely nonsense."
Some critics and Prime Minister Donald Tusk have voiced support for Mr Walesa, saying that the books are politically motivated.
"We need Lech Walesa in Poland as an important authority figure," said Mr Tusk, himself a former Solidarity activist.
The BBC's Adam Easton in Warsaw says the accusations are unlikely to do too much damage to Mr Walesa's reputation.
According to surveys, many Poles say even if he did err as a young man, he is still a hero for what he achieved in the fight for freedom and democracy in the 1980s, our correspondent says.