The Government has been accused of "colonialism" for threatening to sack foreign teachers who have been working in England's schools for years.
Teachers trained outside the European Union will have to achieve qualified teacher status by September or face losing their jobs.
The National Union of Teachers (NUT) criticised the way the rules were introduced, warning that teachers hailing from Commonwealth countries were facing particular problems.
Leonora Smith, a teacher from Sierra Leone, said the employment of experienced foreign teachers on unqualified rates amounted to a "sweatshop" that was growing. Many frustrated with the overseas-trained teacher programme had even resorted to retraining on expensive PGCE courses, she said.
Another delegate, Ally Dworniak, from Redbridge in north-east London, added: "It smacks of colonialism. No teacher can last four years in the system if they are not up to it.
"It implies that overseas-trained teachers' qualifications, particularly those from the Commonwealth, are inferior." Delegates at the NUT's annual conference in Manchester passed a motion calling for an amnesty for all foreign staff who have been teaching in England for more than four years.
Last year, the union convinced the Government to delay the introduction of the rule from September 2007 to 2008.
Hendrina Quinnen, 47, originally from South Africa, has 26 years' teaching experience, and has worked in Birmingham schools for the past eight years.
Ms Quinnen lost her job last year because she did not have fully qualified teacher status and is unable to get supply work. She lives on benefits. This is despite the fact that she received personal praise for her teaching from Tim Brighouse, Birmingham's former education director, who stood in during one of her lessons.