At least 11,000 teachers in London do not have Qualified Teacher Status
Schools in London could face a shortage of new teachers in September as many staff do not have the Qualified Teacher Status (QTS), academics have warned.
London's Institute of Education (IOE) said one in six schools could suffer as teachers, including those with overseas qualifications, do not have the QTS.
From September, all state schools will only be allowed to employ registered staff who have the QTS qualification.
The Department for Children, Schools and Families said "there is no crisis".
At present about 11,000 teachers in London are unregistered, the IOE said.
Without the QTS, teachers cannot register with the General Teaching Council.
The warning came a day after the Conservatives cited government data to show the number of unregistered teachers in England had risen five-fold in 10 years.
Teaching staff need to pass skills tests in English, maths and information technology to obtain the QTS.
Any teacher who has equivalent overseas qualification can join a school as an unqualified teacher and will be assessed for a year before he or she is deemed fit to take the QTS tests.
At present, all foreign teachers are required to convert their qualifications to QTS within four years of arriving.
Professor Peter Earley and Sara Bubb, from the IOE, said: "Although there are 69,200 teachers in the capital's state schools, only 57,773 London teachers have registered and that total includes independent school teachers.
"That is a matter for concern because it looks as if over 11,000 teachers - a sixth of the capital's workforce - aren't registered, presumably because they aren't qualified."
Schools in London already face a staff crisis as the city has double the teacher vacancies compared with the national average and could find it hard to recruit qualified staff in the autumn.
"It is good that the employment rules are being tightened because London children need fully qualified teachers.
"However, the short-term consequences are worrying, if pupils are without teachers," the experts said.
But a spokesperson for Department for Children, Schools and Families said: "There is no crackdown, there is no crisis.
"Overseas trained teachers will still have four years to qualify for English Qualified Teacher Status, the only change is that they won't be able to artificially extend the period by drawing out the course indefinitely.
"We announced this change last summer, so people were aware and have plenty of time to complete their QTS."