BANGKOK — After months of internal debate, members of the party of Daw Aung San Suu Kyi, the long-detained pro-democracy leader, defied Myanmar’s junta by announcing Monday that they would boycott the country’s first elections in two decades.

The move raises questions about both the future of the Burmese opposition and the credibility of the vote.

According to election laws the junta released earlier this month, the decision means that the party that has served as the mainstay of the country’s democratic movement for two decades, the National League for Democracy, will be automatically dissolved. Western governments, including the United States and Britain, had said that Mrs. Aung San Suu Kyi’s participation and that of her party were prerequisites for legitimate elections.

On Monday, U Win Tin, a founding member and strategist for the party, said that more than 100 delegates were unanimous in their decision. “We will ask the people around us not to vote in the election: Please boycott,” he said in a telephone interview. He said that the party would try to continue political activities after it is disbanded. “We will work for the people,” he said.