Part-timers in England and Wales face a postcode lottery with some authorities paying for an hour's less work than others, costing the teachers up to Pounds 22 a day. Four out of five part-timers are women and their numbers have trebled in the past 20 years.
Alan Johnson, the Education Secretary, has asked the pay review body to approve changes which would force employers to give part-time staff, including long-term supply teachers, the same rights as full-timers.
He has also asked the review body to investigate increased pay flexibility for Wales, prompting concerns that teachers there could get different salaries for the same job as those in England.
Unions fear Welsh teachers could get paid thousands of pounds less. They warn that the move could create a precedent for those who want to introduce regional pay in England.
The review body report will also make recommendations on allowances for teachers with responsibility for special needs, pay for excellent teachers and ways to improve science and maths teacher recruitment.
Part-time teachers are legally entitled to pro-rata pay equal to their full-time colleagues but a loophole means employers can use different methods to calculate the number of hours worked.
A survey by the rewards and incentives group, which represents the Government, employers and the social partnership unions, found authorities take a variety of approaches to non-contact time.
John Howson, a recruitment analyst, said: "Some schools, particularly those which are strapped for cash, redefine what constitutes the time they pay for. They will only pay for class contact time. So teachers find they are not paid for assembly or for lunchtime. But they still need to get in early, especially if they want to get a parking space. It's a product of individual schools playing the rules."
Last year Wendy Morris, a supply teacher, received pound;730 compensation from Cheshire county council when a court ruled she was not being paid the proper rate.
She said: "There are teachers who can't find work, and are desperate. So they keep coming back and being underpaid. It's exploitative, unfair and pre-Victorian."
Unions welcomed the proposed changes for part-timers but promised an "immediate campaign of resistance" against a different pay system for Wales.
Chris Keates, NASUWT general secretary, said: "Wales is a low-wage economy and any separation would have a devastating adverse impact on teachers' and headteachers' pay."
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