More than 250,000 qualified teachers no longer work in England's schools, the Conservative Party says.
The Tories say many leave teaching due to government red tape
And nearly 100,000 switched careers between 2000 and 2005 - more than double the number that left in the preceding five-year period.
The Tories say their findings - based on government figures - point to rising numbers leaving the profession because of poor class discipline and red tape.
But Schools Minister Jim Knight said recruitment was "buoyant".
Figures also show that thousands of people who train and qualify as teachers never go on to work in schools and this appears to have increased in recent years.
The government statistics show that of those who qualified in 2000, 2,100 never taught in schools. This rose steadily to 2005 (the latest available), when 7,900 of those who qualified have never taught.
Shadow schools secretary Michael Gove said teaching talent was "going to waste".
Mr Gove said: "Not only are our children not achieving as they should, talented teachers are not where they should be - in the classroom, opening young minds to new horizons.
"With more than quarter of a million gifted professionals no longer in teaching, we have to ask why they've given up on education under Labour."
He said teachers needed to be freed from "government micro-management" to enable them to "inspire and give them the tools to enforce discipline so that schools have access to the widest range of talent".
But Mr Knight said teaching was now "the career of choice for many highly qualified, talented individuals".
He went on: "Ofsted has said this is the best generation of teachers ever.
"Early retirement and churn in teaching is in fact good compared with equivalent professions."
He said: "No government has done more to support teachers".
Mr Knight also cited a Bath University survey of 22,500 British workers which suggested that school, college and university teachers have climbed from being the 54th happiest occupation in 1999 to 11th in 2007.
Meanwhile, a spokesman for the Training and Development Agency for Schools said: "Many qualified teachers decide to take a break from the profession for a number of reasons."
The spokesman said the figures referred to by the Conservatives "do not take account of the fact that up to 30,000 teachers return to teaching at a later date with added industry experience and a new enthusiasm for teaching and learning".
But the general secretary of the NASUWT teaching union, Chris Keates, said Mr Gove's figures were probably about right but his claims about reasons for teachers' inactivity were wrong.
She said a pool of inactive teachers had always existed and most joined it because of career breaks or changes.
She added: "For the majority of those who leave now, evidence shows it is a positive choice."