Several thousand students in their final year of teacher training will be forced to work until they are 65 because of changes to pension rules and a shortage of jobs.
A deal struck last week between the Government and unions will allow existing teachers to continue to retire at 60, but raise the retirement age to 65 for those who join the profession after September 1 2006.
Union leaders were jubilant about last week's deal, which was widely seen as a victory for public-sector workers. But they admitted this week that the changes will penalise teachers who qualify next summer and are unable to find jobs until after the start of the autumn term in 2006.
Those who choose to take time out between completing their training and starting work will also be affected.
A study by Buckingham university's centre for education and employment research found that two-thirds of those who qualified as primary teachers last year were still jobless in March.
John Howson, a recruitment analyst, said the situation is likely to be worse next year because new trainees will be added to those already unable to find work.
"There is a long tail of people who are going to be caught by this," he said. "It would be fair to say it will affect several thousand."
Fiona Robinson qualified as a primary teacher in June 2004, but has so far been restricted to supply work for Stockport education authority because she has been unable to find a permanent post.
She said: "Although I joined the pension scheme when I trained on the graduate teacher programme, I am having to consider how long I will continue looking for a job.
"This situation will make things worse. People will decide to stop looking if they think their pension is going to be affected."
The pensions deal agreed last week was hailed as a victory for teacher unions, which had threatened industrial action over plans to raise the retirement age of all teachers to 65. Under the agreement, existing members of the teachers' pension scheme will continue to be eligible for retirement at 60, although they will be given greater incentives to continue to work after that date.
Details of these incentives and of changes to ill-health retirement and dependants' benefits will be discussed in further talks between the Government and unions, which are expected to start in the next two weeks.
Gerald Imison, deputy general secretary of the Association of Teachers and Lecturers, said unions would take up the issue of teachers who missed the deadline to retire at 60.
He said "We have made a significant achievement for existing members of the profession but we do not want that to come at the expense of their future colleagues."
He said those who begin supply teaching before September 1, 2006 could still lose out because most such work is not pensionable.