Lech Walesa has left the Solidarity trade union movement he founded and led during its struggle against communism in the 1980s.
Mr Walesa said he would not attend the ceremonies to mark the 26th anniversary of Solidarity later this month.
He said he was in disagreement with his former political allies, President Lech Kaczynski and his twin brother, Prime Minister Jaroslaw Kaczynski.
A year ago Mr Walesa hinted he was planning to quit Solidarity.
"This is no longer my union. This is a different era, different people, different problems," he said during celebrations to mark the 25th anniversary of the movement last August.
Anniversary boycott
Mr Walesa, who won the 1983 Nobel Peace Prize, has not been a member of the trade union since the beginning of the year, it was revealed on Tuesday.
"He has not been paying membership fees since the end of last year," a leading Solidarity member, Jerzy Borowczak, told the AFP news agency.
Mr Walesa said he did not like the policies of the Kaczynski brothers and would boycott Solidarity's anniversary celebrations so he would not have to appear with them, the BBC's Adam Easton reports from Warsaw.
Mr Walesa once fired the twins when they worked as his advisers while he was president in the 1990s.
"His approach is to first destroy and then think about what to build," Mr Walesa said last month of the country's current president, Lech Kaczynski.
Main disagreement
The main point of disagreement is the new policy to make public all the files of the former communist secret police.
Until now only members of the government and parliament had to declare whether they had any connection to the former security services.
"I didn't like their conspiracy theories. They were always suspecting people," Mr Walesa said about the Kaczynski twins.
Critics warn that the so-called transparency legislation might turn into a witch-hunt.
More than half a million Poles, whose names appear in the secret archives, could face vetting.