Two conflicting policy factors affect demand for teachers. Pupil numbers are falling. There will be 62,000 fewer pupils in 2016 than at present.

Unless class sizes are improved, this might mean losing nearly 3,000 teaching posts. At the same time, 35 per cent of teachers in Wales will retire over the next 10 years.

For some time, many trainees have not been able to find teaching posts in Wales, and have had to migrate to England. When teachers were scarce there, this was an inconvenience rather than a problem. But as falling rolls and improved recruitment have changed the picture, jobs in England are more difficult to come by.

As a result, many of those who train in Wales are no longer guaranteed a teaching post. The General Teaching Council for Wales says only 215 of the 812 primary teachers who qualified in 2003 had completed their induction one year later. Even among secondary trainees, less than two-thirds had finished their induction. This imbalance cannot continue.

Education and lifelong learning minister Jane Davidson has ordered cuts in intake targets for primary trainees of 5 per cent for 2005, with a warning that up to 115 places may go by 2006. But to reduce trainee numbers further will imperil the remaining providers in Wales.

So what should be done? I have advocated a reopening of the early-retirement scheme on a matched basis. For every unemployed NQT who trains in Wales, a teacher in their late fifties could be offered early retirement. This programme could be self-funding, given the difference between their respective salaries.

This response offers a longer-term solution. The risk of doing nothing is that, with top-up fees on the horizon and universities being able to charge Pounds 3,000 for teacher-training courses, potential trainees will shun the profession for fear of ending up without a job.

Just as the retirement bulge arrived, teaching could hit another crisis, this time self-inflicted.

And a perception of teaching as an overcrowded profession might spread to the still hard-to-recruit areas of men in primary, Welsh-medium teaching, and certain secondary subjects. This would be a disaster.

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Professor John Howson is a director of Education Data Surveys