Voice, a non-striking classroom union representing 38,000 teachers, support staff and childcare workers, will hear concerns about the new Masters in Teaching and Learning (MTL) at its annual conference next week.
The Government hopes that all new teachers will eventually study for the qualification, which will be offered to the first cohort of 3,000 teachers next year.
But a motion at the Voice conference calls for a rethink, saying the new qualification is unnecessary, politically motivated and could lock teachers into a narrow programme of state-approved study.
Nardia Foster, a psychology teacher at a north London girls' school, said adding the masters qualification to the expectations on an already highly trained workforce was a case of "moving the goalposts".
"The Government says it will increase professionalism, but it is only the Government saying that we are not professional enough already," said Mrs Foster. "This push is not coming from those within teaching."
She said a straw poll among staff in her own school revealed that few agreed it would improve teaching.
Mrs Foster, who has a masters degree in the psychology of religion, said she believed the drive to encourage young teachers to complete a specific teaching masters also raised vital questions about the purpose of education.
"Everybody has the right to develop themselves, and I did my MA because it was something I wanted to do," she said. "But you ask the question, `Is education about the development of the person or for the Government to control what people do?"
During the conference, Mrs Foster will also raise concerns that older, more experienced teachers could eventually miss out on jobs if they do not have the new masters qualification. An outline of plans for the MTL said there would be "no direct link" between the masters degree and pay and career progression, but that it would become "a factor" in recruiting as more staff gained it.
The Universities' Council for the Education of Teachers expressed concern last month that the new MTL risked becoming a watered-down version of the traditional academic qualification, or a "masters in teaching lite".
Graham Holley, chief executive of the Training and Development Agency for Schools, has previously described the qualification as "vital" in raising standards and narrowing the achievement gap for children and young people. He said it would make a "dramatic contribution" to improving the quality of teachers.
Another motion at the Voice conference, which starts in Daventry, Northamptonshire, on Monday, will call for simplification of the range of qualifications now available to secondary pupils.
Simon Smith, a design and technology teacher from Sweyne Park School in Rayleigh, Essex, will argue that the new 14-19 diplomas add to an already baffling array of qualifications, including A-levels, Btecs, NVQs, DIDA and the international baccalaureate.
Some 100 members of Voice, previously the Professional Association of Teachers, will attend the conference, which will also hear from Andrew Adonis, the junior minister for schools.