The problem is likely to be exacerbated by an increase in the number of teacher training places to combat the recent teacher shortage.
A projection of pupil numbers prepared by the Department of Education and Skills last year revealed that the primary school population fell by 98,000 between 1998 and 2002.
A further fall of more than 400,000 primary pupils is predicted by 2012, reducing pupil numbers to a figure last seen in 1988.
John Howson, a director of Education Data Surveys and a visiting professor at Oxford Brookes University, said: "Half a million fewer primary school pupils could mean the choice between either smaller classes or around 20,000 fewer primary school teachers. For the secondary sector, it could mean a need for some 16,000 fewer teachers."
He said job losses could be off-set by smaller class sizes, but added that in some areas that might not be enough to deal with the surplus, and newly-qualified teachers could have problems finding jobs locally.
"Falling school rolls affect budgets," he said. "I am starting to get nervous that there is going to be a crunch where people find it difficult to get a job."
The number of primary pupils peaked at 4,614,000 in 1998 but is predicted to drop to 4,108,000 by 2012. A Teacher Training Agency spokesman said:
"The DfES has a very complex model in working out how many teacher training places it should fund. School population estimates, teacher age, and retirements are all taken into account when fixing the number of placements it feels it needs."