Eng Chhay Ieng, who had been the secretary-general of the Sam Rainsy Party, has chosen to resign, following disappointing results in July's election
Eng Chhay Ieng, who had campaigned in Battambang province, said that the election had only resulted in two seats for the party there, the same number of seats as in the previous government.
The resignation comes as the Sam Rainsy Party continues to boycott July's election results.
Eng Chhay Ieng said he was stepping aside to make room for others, but he would not leave the party. He did not know who would replace him, but he said the party has many people capable of doing the job.
Eng Chhay Ieng had been criticized within the party and publicly by Prime Minister Hun Sen for excessive gambling. However, he said Tuesday the gambling accusations were not a reason for his resignation, and that he had now stopped the habit.
Eng Chhay Ieng resigned from his position in 2005, following gambling accusations, but he took up the position again in 2006.
Chea Vannath, former director of the Center for Social Development, said the resignation would bring two positive developments to the opposition party.
Firstly, it will reduce internal tensions within the party, she said. Secondly, Eng Chhay Ieng's willingness to resign rather than struggle for power would set a good example for others in the party.
Government spokesman Khieu Kanharith said Tuesday the Sam Rainsy Party had suffered weakened support because of its election strategy, not because of Eng Chhay Ieng.
Opposition leader Sam Rainsy could not be reached for comment.