The party's education spokesman, Don Foster, said the numbers of teachers being employed on "holidaymaker visas" showed the extent of the teacher recruitment crisis.

"Three years ago, working holidaymakers could be thrown out of the UK for pursuing a teaching career full-time," he said. "Now the education service in London is dependent upon them.

"Concern about the numbers of agency staff working in the health service is widespread, but there should be rather more concern about the extent to which temporary staff are working in our schools."

Temporary teachers from overseas were unlikely to provide continuity in the classroom, said Mr Foster.

"Many are highly effective teachers, but they return home when their visas can no longer be extended."

Mr Foster is calling on the government to provide information on the number of teachers employed on holidaymaker visas in the UK.

His claim that there are up to 1,000 overseas teachers in London is based on an estimate by Timeplan Education Group, the largest independent supplier of temporary teachers in the capital.