This is despite ex-chief inspector of UK schools, Chris Woodhead, estimating some 15,000 are not up to the job.
Some bad teachers are moved between schools, rather than having their competency challenged, it has emerged.
Teaching unions dispute the claims. The General Teaching Council for England, which investigates complaints, says the number of poor teachers is "not clear".
However, the GTC admits the suggestion that the 18 struck off represented the total number of incompetent teachers in the system is not credible.
Two years ago, its chief executive Keith Bartley said there could be as many as 17,000 "substandard" teachers among the 500,000 registered teachers in the UK.
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