The General Teaching Council for England has failed to make a positive impact on teachers and needs to be scaled down, the Audit Commission has said.
A review carried out by the commission for the GTC found that a majority of teachers were "neutral or negative" about the organisation.
It said: "There is a view that the council is duplicating policy development and research work that is now provided on a larger scale through the Department for Education and Skills and related agencies."
It noted that public criticism of the organisation by one teaching union was "giving an impression of internal division that is harming the council's reputation". Although it did not say which one, it is believed to be the National Association of Schoolmasters Union of Women Teachers.
The review said that the council "has not yet made the impact it aspired to with key stake-holders, including the Government and the majority of the teaching workforce". Its relationship with the Government was "less productive than it should be".
The GTC also needs to enhance its public image. "Some of the council's public description of its research and policy-development work lacks coherence and is over-detailed by comparison with its accounts of its registration and regulatory functions," the report said.
The commission said that the GTC should reduce its 64 members and that union representation should be scaled down.
However, the report noted that the council was "well-managed internally and is making steady progress with its regulatory and registration processes".
It said: "There is an open and enthusiastic culture and a strong improvement ethos."
Carol Adams, the GTC's chief executive, said a working group would be set up to consider the findings, but any changes to the size of the council would require legislation.
Steve Sinnott, general secretary of the National Union of Teachers, said it was ridiculous to consider removing union nominees.
Chris Keates, general secretary of the NASUWT, said: "If the GTC concentrated on its regulatory functions, its standing and reputation among teachers would improve drastically."
The report was published as a dispute between the GTC and the Government escalated over the registration of teachers working in academies. A meeting of the council voted unanimously this week to urge ministers to bring academies into line with maintained schools.
In a letter, Stephen Twigg, the school standards minister, said that academies, as registered independent schools, are "not bound by the statutory requirement for their teachers to be registered with the council". He said it was up to governing bodies to determine whether teachers should register.
Judy Moorhouse, GTC chair, said teachers who had been disciplined by the body might turn up at academies, placing pupils at risk. "Part of the reason for creating the GTC was to prevent this happening," she said.
Margaret Morrissey, of the National Confederation of Parent Teachers Associations, said: "Parents will not understand why this exemption exists and they will not consider academies as being like other independent schools because clearly they are not."