New teachers trained in Wales are being lured to England with gimmicky gifts and better options for career breaks and retirement, it has been claimed.
Meanwhile, proposed cuts in teacher-training places in Wales should be rejected for fear of leaving Welsh schools short of new recruits in future years, unions have warned Assembly members.
The London borough of Greenwich was named as an English council offering attractive packages to trainee teachers in Wales by the National Union of Teachers Cymru.
Dr Heledd Hayes, the union's education officer, last week told the Assembly's education, lifelong learning and skills committee: "We had coal exports in Wales - now to our credit we are exporting teachers. But it is now time to be more creative to keep teaching talent in Wales."
But Dr Philip Dixon, director of the Association of Teachers and Lecturers Cymru, warned against Wales becoming too insular, saying its teacher exports should be seen as a strength.
And William Graham, Conservative education spokesman, said: "I am horrified at any suggestion that could lead to us becoming 'fortress Wales' - even if it means keeping our teachers."
All the unions were against plans to cut primary teacher-training places by half and secondary places by a quarter.
The proposals, made by the Furlong review, commissioned by the Assembly government, have been accepted in principle by Jane Davidson, education, lifelong learning and skills minister.
In his report, Professor John Furlong, from Oxford university, said Wales could not justify training teachers it did not need, especially as it now has to pay for courses itself.
Last year, hundreds of Welsh-trained NQTs struggled to find permanent posts in which to complete induction, and falling pupil numbers mean even fewer teachers will be needed in future.
But unions claim the review did not take account of Wales's ageing teacher population and the alleged brain-drain of new teachers to England.
And they warned that Assembly government initiatives, including reform of the 14-19 curriculum and introduction of a play-based foundation phase for three to seven-year-olds, would mean more demand for new teachers in future, not less.
Geraint Davies, secretary of teachers' union the NASUWT Cymru, called for a national syllabus for teacher-training colleges in Wales. NQTs coming out of training are almost ignorant of new policies such as the foundation phase, he said.
Dr Dixon applauded the honesty of the Furlong report but said: "We can't afford to get this wrong."
He also supported Furlong's proposals for a new "pre-professional" degree, which have been rejected by the minister. These would provide a solid grounding for people wishing to go into education-related jobs.