More than 400,000 qualified teachers are no longer working in the classroom, according to new figures published today.
Just over 404,600 fully trained teachers under the age of 60 are no longer teaching, compared to around half a million still actively working in England and Welsh schools.
A closer analysis of the figures also shows that 25,000 people who qualified as teachers since 2000 never entered a classroom.
The breakdown has angered the Conservatives who say measures to improve school discipline are necessary to persuade staff to stay in the front line.
"This is a tragic waste of talent that is costing the taxpayers millions of pounds every year," said Michael Gove, their schools spokesman.
"We are training thousands of new teachers every year only for many of them to leave within a few years of becoming qualified. At the same time, there are literally hundreds of thousands who are qualified but have turned their backs on the profession."
The figures come after a survey by a teachers' union, the Association of Teachers, showed that two-thirds of teachers had considered leaving the profession, because of aggressive pupils, verbal abuse and the threat of violence.
In a separate survey by the same union, more than half of primary school teachers said behaviour had deteriorated over the past five years.
Mr Gove added: "The Government must take responsibility for driving so many experienced professionals out of the classroom by tying their hands in red tape and watering down their powers to keep order."
He said the Conservatives would abolish the right of pupils to appeal against exclusions – thus giving headteachers more control over discipline in their schools.
They would raise the status of the profession by making entry requirements more rigorous to ensure high quality graduates enter the profession.
Schools Minister Vernon Coaker said it was "a New Year own goal for the Tories to be spinning this story".
"2010 is the year the Tories want to cut funding to schools, if they are elected, within 50 days," he added.
"The Tories are deliberately ignoring the fact that thousands of these qualified teachers are still teaching – in higher and further education, independent schools, private tutoring and part-time teaching."
He said former teachers were being targeted for a return to the classroom, with the result that around 10,000 a year were doing so.
Teacher recruitment experts said they were reaching their targets in every subject for the first time – largely as a result of older people switching to the job as as second career, particularly in maths and science. Career switchers are generally thought to be more likely to stay in their second-choice profession, they added.
Graham Holley, the chief executive of the Training and Development Agency for Schools, which is in charge of teacher recruitment, said second careerers wanted to switch to work that they were "likely to find more fulfilling, more rewarding, more challenging and where they can indulge their love of their subject, which they can't do in mindless jobs in business".
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