The comment by Rex Phillips, Wales organiser of the NASUWT, comes with growing condemnation of a 37 per cent increase from pound;33 to pound;45 for the compulsory GTCW registration fee.
The council this week blamed the hike on an increase in professional conduct cases. But the NASUWT's general secretary Chris Keates told teachers in Cardiff that it was totally unjustified.
"The GTCW seems to be constantly looking for new work rather than concentrating on getting its regulatory function right," she said. "Teachers will resent having to subsidise activities which they feel add little value to their life."
Last week Dr Philip Dixon, director of the Association of Teachers and Lecturers Cymru, accused the Assembly government of being petty over its refusal to reimburse the pound;12 rise. But Ms Keates said the government did not have devolved powers in relation to teachers' pay.
Gary Brace, chief executive of the GTCW, said: "The new fee will remain at this level for the next three years. The increase is absolutely necessary for us to remain financially sound as an organisation."