The ASTI no longer reflects the views of thousands of teachers and is largely controlled by a small, well-organised group, according to a former executive member.
Writing in today's Irish Times, Mr Joe Coy says there is "widespread disillusionment among people like myself who perceive, rightly or wrongly, that the ASTI no longer represents them".
He writes: "We disagreed with many of the activities carried out in our names over the past four years . . . We disliked some of the tactics used to fight a futile pay battle. We cringed at meetings when speakers with legitimate points of view were shouted down or jeered at.
"We deplored the disrespect shown to visiting speakers such as the Minister for Education. We were saddened to see our traditional support among the public draining away."
He says the union needs to "retain the respect and status on which all good teaching depends. We need to behave in a rational and reasonable manner. In that respect we could learn a lot from the INTO which, through diplomacy and clever tactics, gained a lot more for its members. The public doesn't like to be hectored."
Mr Coy says the increasing power of a small group within the ASTI has been helped by a "dated and dying" branch structure. He says branches with several hundred members can be dominated by the dozen or so who bother to attend meetings.
"The average attendance is between 10 per cent and 20 per cent. Consequently the voice of the 'silent' majority is rarely heard."
Speaking yesterday, Mr Coy said he was not in the least surprised by the latest figures showing a decline in ASTI membership.
"Where teachers have a choice such as in community and comprehensive schools they are opting out of the ASTI and joining the Teachers' Union of Ireland," he said.
Mr Coy, a teacher with 30 years' experience from Tuam, Co Galway, is a former member of the ASTI Central Executive Committee.
Meanwhile, the Minister for Education, Mr Dempsey, is expected to be widely criticised when he addresses both the INTO and ASTI conferences today. Last night, the INTO president, Mr Seán Rowley, said the crisis in special education was deepening despite the Minister's declaration that this area was his main priority. He said 400 teachers could be recruited immediately "for the price of the equestrian centre at Punchestown".