It has written to the second largest teachers' union, the NASUWT, asking it to stop making public statements against the regulatory body over the way it is run and funded.
In turn, the union asked its members to complete an online questionnaire about their perceptions of the organisation which, it said, will be used to inform its campaign against the GTC.
Nearly 7,000 people responded in a week, with 88 per cent saying they did not support the council. However, Carol Adams, GTC chief executive, said the survey asked leading questions.
On its website, the union says some of the GTC's work duplicates that of Government bodies, cannot be considered value for money and could constitute a misuse of public funds.
Ms Adams said this week that the claims about financial misuse were so serious that the council has engaged its lawyers to act. She said: "The constant criticism and unwarranted attacks on the integrity and scope of the council's work cannot go unchallenged."
Chris Keates, NASUWT general secretary, described the legal threat as a hysterical reaction. She said: "I accept that the GTC has to have a wide remit, but there is nothing mandatory about some of the work it is doing, such as research and continuous professional development."