The National Union of Teachers has written to Alan Johnson, the Education Secretary, pleading for six months more to give overseas teachers time to gain the now-obligatory qualified teacher status.
From September, overseas-trained teachers, predominantly from Australia, New Zealand, South Africa and Canada, are allowed to work only four years without the English qualification.
One teacher, Renee Mason, said her partner was still in Britain but she had been forced to return home to Australia with their daughter because she was unable to get her qualified status in time. "Teachers are not just heading over to England for a holiday, to make money or for a boozy experience,"
"The most frustrating thing is that many teachers did not deliberately skip QTS, but were simply in England too short a time to have it come across their radar or, more worryingly, had the information kept from them by agencies or were told not to bother."
Luisa Longone, 29, who teaches at Earlsmead primary school in Tottenham, north London, plans to return home to New Zealand in October, because the qualified teacher status process is too convoluted and intensive.
"In September, there will be huge numbers of well trained, hard working, English speaking Commonwealth teachers who will be out of a job," she said.
"What upsets us is that if you come from the European Union yet do not speak any English, you can automatically get QTS."
While there is widespread agreement that overseas teachers should be required to gain qualified teacher status, the NUT says the Department for Education and Skillshas failed to sufficiently publicise the changes. Tony Brockman, a London union official, said the problem is exacerbated by most training courses being full.
The rules have changed several times in the past six years, but there has always been a loophole allowing overseas teachers to keep their work permits provided they are training for their qualified status.
A DfES spokesman said schools had received "ample" warning of the change and the department had sent out annual reminders to the sector.