The government initiative to recruit more business professionals into teaching is being undermined by poor training, education watchdog Ofsted has warned.
Although the quality of professionals who switch career is high, many are being let down by inadequate training and are teaching badly as a result, according to an Ofsted report.
The government began recruiting mature professionals into education in 2003 under the Graduate Teacher Programme (GTP) - mainly to fill the gap in mathematics, science and language teaching in England.
The report says that 20% of lessons given by trainees on the programme had "unsatisfactory" features and nearly half failed to stretch their pupils.
A third of the 109 bodies overseeing the scheme had "significant weaknesses" in the way they trained secondary school teachers to teach specialist subjects.
"There was a strong link between the quality of teaching that the trainees achieved and the quality of the training they received," the report says.
"Despite GTP trainees' high level of commitment, they did not attain high standards of teaching when they received weak training."
Graham Holley, head of the Training and Development Agency, which is responsible for teacher training, admitted that more needed to be done.
The agency runs an improvement programme for training bodies, he said, "which focuses on the areas highlighted in the report, to ensure every trainee receives the help they need".