Some overseas teachers are exploiting a legal loophole that allows them to work in England's schools without relevant qualifications, the Government has warned.
Quirks that still exist because of devolution make it possible for them to get "prior recognition" as a teacher in Scotland or Northern Ireland with no teaching experience.
This counts as qualified teacher status (QTS) if that person then gets a job in England, while a candidate applying directly to work in England would have to teach in a school here for 60 days first.
The Department for Children, Schools and Families is consulting on changing the regulations so that overseas-trained candidates must also work for 60 days to get QTS in Scotland or Northern Ireland.
But the NUT fears this might hamper those who qualify in the two countries from finding first posts elsewhere in the UK.
Christine Blower, NUT general secretary, said she was worried the Northern Ireland and Scotland changes would lead to a "further layer of unease" and those training there would be "viewed less favourably than a teacher who qualified in the EU" when it came to finding first jobs in England.
But Darren Northcott, assistant secretary of the NASUWT, said he viewed the changes as a "tidy up" of legislation. "I can't see many overseas teachers doing that much investigative work that they know they can apply elsewhere in the UK for QTS in order to work straight away in England, so I doubt there are many cases of this happening," he said.
Other changes proposed for QTS include the possibility of getting the qualification purely by assessment, without any training, for those who are already experienced graduate teachers. The assessment will be managed by the Training and Development Agency for Schools.
Those working in British service schools will now be able to get QTS based on their experience as well.
The changes also remove Guernsey's unusual age limit of 24 for teacher training in order to comply with equalities legislation.
Results of the consultation and the DCSF's response will be published by late autumn.