School Standards Minister David Miliband has rejected accusations that A-levels are getting easier.
David Miliband attacked the "national sport" of scorning success
Mr Miliband said that it was a "national disease" to interpret pupils' increased success as evidence of a lowering of standards.
With no evidence of "dumbing down", he said he "condemned this national sport of talking down young people".
But he promised to investigate claims made by an exam marker that grades were being inflated.
Mr Miliband, speaking on BBC Radio 4's Today programme, said that there was independent evidence to show that the "rigour" of exams was being maintained.
Exams were run and regulated by independent bodies, he said and people "should not fall for this national idea that if more are doing well it's a problem".
"I'm not saying that young people are getting cleverer, it would be absurd to say that the IQ distribution has improved from one generation to the next.
"What I am saying is that schools are getting better at realising the potential of young people," said Mr Miliband.
But the minister said there would be an investigation into allegations made by an examiner, writing in The Times, which claimed that she had seen first-hand evidence of "grade inflation".
Patricia Voute reported her experience of marking A-levels this year, where exam entries which she had failed were being revised upwards to D or E grades - and where papers she had viewed as C grades had been awarded A grades.
She said the standard of papers had been so poor that she had believed that a third should have failed.
"The A-level system has been bankrupted by grade inflation," wrote Patricia Voute.
Mr Miliband said that there was ongoing independent scrutiny of A-level standards - and that such complaints would be investigated.
"I take their concerns extremely seriously and they will be investigated by the independent authority that sets exams," said Mr Miliband.