The government scheme to get City high-flyers into the classroom within six months would be too short to produce high-quality teachers, say experts who run existing school-based training.
While it is possible for trainees to complete schemes such as the Graduate Teacher Programme within six months, only a "handful" actually do, and most need a whole year, the National Association of School Based Teacher Trainers said.
The organisation is worried that the courses they have developed could be threatened if the fast-track route promoted by Schools Secretary Ed Balls last week goes ahead.
Martin Thompson, the association's chairman, said only those with prior experience of education finish school-based courses quickly - which conflicts with the Government's vision for the new training scheme.
"Only those with a significant experience of being in the classroom end up finishing the GTP before the end of the school year," he said.
"We see many high-flyers start out thinking they might, but they always elected to have more time once the course had started.
"We don't want to deter people from transferring from other jobs into teaching - we just want them to know it's not easy, and far more of a challenge than they might have imagined.
"This organisation supports the provision of a number of distinctive routes leading to the provision of high-quality teachers in schools already accessible to individuals wishing to change careers."
The Department for Children, Schools and Families was unable to say when details about the fast-track course would be available.
Letters, pages 36-37.