A no-deal Brexit would end the automatic recognition of European teaching qualifications and the sharing of details of sanctioned teachers between states, the government says.
The Department for Education today said it is “intensifying and accelerating no deal planning” as the deadline for Britain to leave the European Union approaches without an agreement in place.
Geoff Barton, general secretary of the Association of School and College Leaders, said the information made "worrying reading".
The DfE guidance says: “In a no deal scenario, the current system of reciprocal recognition of professional qualifications between the EU, EEA, EFTA and Switzerland and the UK will not apply after 29 March 2019.”
However, it says this would not affect those who have already had their qualifications recognised in the UK or have applied for a recognition decision before 29 March 2019.
It adds: "There will be no retrospective change for people who have already had their EU, EEA EFTA and Swiss professional status and qualifications recognised and been awarded Qualified Teacher Status in England.”
The DfE said it will publish details of a new system to allow teachers with EU EEA EFTA or Swiss qualifications to gain recognition after Brexit “shortly”.
Mr Barton said: "Schools in the UK recruit a large number of EU nationals as teachers and this supply line is particularly important in terms of modern foreign languages where there are acute teacher shortages.
"It is important that any change to the current system of professional recognition does not create unnecessary barriers or bureaucracy which hinders that supply line.”
The DfE will also update its safeguarding guidance as a no-deal Brexit would end the requirement for EEA professional regulation bodies to share details of teachers who have been sanctioned.
Mr Barton described anything that hinders the ability of schools to check the professional competence and conduct of a prospective teacher as "clearly unwelcome".
He sought to reassure the public that schools "will always take all steps to verify the suitability of any new teacher", but added that "last thing they need is a retrograde step which makes that process more difficult".
Teaching is covered by the EU’s ‘mutual recognition of professional qualifications’ directive, so a teacher from an EEA member state is deemed qualified to work in the UK.
DfE statistics showed the number of overseas teachers applying for QTS in England slumped 25 per cent last academic year.
Tes also revealed in June that international teachers from outside the EU working in British schools were being forced to leave their jobs because they couldn’t get visas.
In a victory for the #LetThemTeach campaign, the DfE has pledged to consider giving international teachers higher priority for UK visas.