Last week, The TES reported that supply teachers' pay rates can vary by up to pound;45 a day depending on which agency they work for.
Such staff cannot demand the same pay and conditions as full-timers, because they are not covered either by the teachers' pay and conditions document, or by the latest European employment directive.
The TES has called for a new charter for agency recruits, along the lines of the "Bill of Rights" drafted recently by American supply teachers.
Peter Smith, general secretary of the Association of Teachers and Lecturers, said: "All the thrust of recent employment legislation has been that part-time workers should be paid pro-rata on the same basis as other employees.
"I think that's absolutely right. Anybody who thinks that supply teacher are mere waifs and strays, to be ignored, is wrong, and is missing the fact that many of them are performing vital jobs in keeping our schools going.
"The TES is right to raise this issue, and we are right behind it."
Eamonn O'Kane, deputy general secretary of the National Association of Schoolmasters Union of Women Teachers, said he was particularly concerned that most agencies did not contribute to teachers' pensions.
"We would welcome any campaign that helps to improve matters, particularly on the pensions issue," he said.
The Government is now advertising a contract to develop a distance-learning package for supply teachers, to include material on effective classroom and behaviour management techniques.
Do you feel a supply agency you work for could be offering a better service? The TES has been conducting an online debate on the subject, through our website. Go to www.tes.co.uk to take part.