Around one in every 33 teachers working in English schools is unqualified or did not train in the UK, according to statistics released this week.
There has been a fivefold increase in the number of unqualified and overseas trained teachers working in English schools since 1997.
There are now 14,500 overseas trained, unqualified instructors, or trainees without qualified teacher status, according to government figures released this week.
And for the first time since 1998, the number of full-time qualified regular teachers has fallen, by 1,300. Part-time numbers are up 1,500 on last year. The number of qualified regular teachers overall is up 11,500 since 1997.
The Department for Education and Skills said most overseas trained teachers are highly qualified and from English-speaking countries.
But Damian Green, Conservative education spokesman, said: "These figures expose the falsehoods in the Government claim that it has employed thousands of extra teachers.
"They show that between 1997 and 2003 half the extra teachers were without qualified teacher status."
John Howson, a recruitment analyst, suggested ministers' pledge to recruit 10,000 more teachers between 2001 and the next election was "looking a bit dodgy". The DfES said it had already hit the target.
Phil Willis, education spokesman for the Liberal Democrats, said: "Given the current budget crisis, schools are likely to use more unqualified staff as a way of balancing the books."
The figures show only slight changes since provisional data was first released in April.