But Christine Blower, general secretary of the National Union of Teachers, said: "There will be some, in particular those with special educational needs, for whom achieving a C grade will be simply impossible."
Children who do not get a C grade or above in their English and maths GCSEs will have to continue studying the subjects until they pass.
Previously, students could drop maths and English from their timetables at 16, but from this term, teenagers who do not make the grade will resit the tests until they pass or leave full time education - extended to 17 from this year.
The move comes after employers warned government that school leavers needed to improve their basic skills.
Education Secretary Michael Gove has said English and maths are the subjects 'employers demand before all others'. Last year, more than a quarter of a million 19-year-olds did not have a C grade in either.
Skills minister Matthew Hancock told the BBC that the new measures were about continuation of learning, rather than endless re-taking of exams.
"For those who fail to get a C at GCSE, it's a huge impairment to their future life, their ability to participate not just in work but also as a citizen," he said. "The crucial thing is that they keep learning and if they can get up to that GCSE standard, that's terrific."
The changes will take place from next term.
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