GCSE exams should be "thoroughly overhauled," the chief schools inspector in England has said.
Sir Michael Wilshaw's comments come after the refusal of exam regulator Ofqual to order exam boards to regrade this summer's GCSEs in English.
The regulator acknowledged grade boundaries had changed between January and June, but offered only resits.
Sir Michael said the row was a "really good opportunity" to examine whether examinations were "rigorous enough".
Teaching unions are angered that many pupils now face the prospect of resitting exams rather than having their papers automatically regraded.
They claim those who sat the exam in June were at a disadvantage, compared with those who had sat English GCSE earlier in the year, because the summer's exams were marked over-harshly after Ofqual told exam boards to keep an eye on grade inflation.
Sir Michael, who is head of Ofsted, told the Andrew Marr programme that the matter was an issue for Ofqual.
But he added: "This is a really good opportunity for our system and the secretary of state to look at our examination system and ask whether it is rigorous enough, whether it's credible enough, whether what's happened over the last few years in terms of resits, early entries and the modular approach to to examination is actually raising standards".
He said that the issue was "not really summer against January and what happened last year as opposed to this".
"It is whether our examination system stands up with the best in the world.
"One of the worries I have and I know other people have, is that our standards are falling in relation to other countries in the rest of the world.
"English is the world language, it is the business language. We know that we have fallen from 7th in reading to 25th in the world. In maths from 7th to 28th. That is not good enough. We've got to look at what is happening in the rest of the world.
"Our youngsters, when they leave school, will be going into a global marketplace, they have to compete not just against competitors here but the rest of the world."
Sir Michael also said that the end of the "satisfactory" grading for schools was "not window dressing, it is not just changing the terminology".
Ofsted is to replace its "satisfactory" grading with a "requires improvement" grading.
Sir Michael added: "Two-thirds of our schools are good or better. We have got a third of schools, 6,000 schools that are not good, that are satisfactory and below.
"We have to make sure that schools know they have got to get to good soon as possible. We have given them a prescribed period of time, up to four years, in which to get to good.
"Inspectors will be spending more time in lessons now than ever before. We won't be looking at as much paperwork as we used to do. It is about being in the heart of the school, looking at what really matters, which is what is happening in the classroom."
He also said that the "big issue" is that head teachers have to lead teaching. "They are not head administrators or organisers or head business managers, they are head teachers."
Meanwhile, Prime Minister David Cameron has dismissed criticism of the fall in GCSE results in an article in the Mail on Sunday.
He said he he planned to reverse "dumbing down" and he refused to "cave in" to teaching unions who prefer to "pretend standards are rising each year".
"'All must have prizes' is not just patronising, it is cruel - and with us it is over," he added.
And Mr Cameron said there would be "no more excuses for failure in schools, no more soft exams and soft discipline".