Exams boards were ordered to tighten up GCSEs in four subjects amid fears that it was becoming easier for pupils to pass on Friday.
The exams regulator Ofqual announced that it is making changes to GCSEs in English literature, maths, history and geography to ensure that students study the whole curriculum.
It is understood that it comes after concerns were raised that pupils were sitting papers after only studying topics that were likely to come up in the exams, rather than the entire course.
Ofqual chief executive Glenys Stacey said: "We are tightening GCSEs in these key subjects to make sure students cover the whole curriculum.
"We want our young people to have the best possible educational experience, with qualifications that prepare them for the future.
"The exam boards have welcomed this steer from the regulator and are to look again at these qualifications and how the rules are interpreted to make sure that young people taking them have to study an appropriate range and depth of the subject."
It is understood that today's announcement is part of an attempt to move away from teaching to the test and to encourage breadth of study with pupils learning everything in the course.
Ofqual said that GCSE geography will be improved for first teaching this September, and maths from this November, with history and English literature papers changed from September 2013.
In December the Daily Telegraph released video footage of a conversation between an undercover reporter and a chief examiner at Edexcel who claimed the company's GCSE geography tests were not as difficult as those from other exam boards.
Speaking at an Edexcel seminar in Birmingham in November, Steph Warren said teachers should choose their exam because "you don't have to teach a lot".
She added: "There's so little (in the exam) we don't know how we got it through."
Edexcel later announced that an internal review, analysing data from all five awarding bodies for GCSE geography, history and English had found that candidates were "no more or less likely" to achieve certain grades with its exams compared with other boards.
It said that it had also assessed the "breadth and depth of the exam specifications" and was confident they were of the appropriate standard set by Ofqual.
Appearing in front of a select committee this week, Mark Dawe, chief executive of the OCR exam board suggested that examiners which have seen questions relating to future exams could be banned from attending seminars with teachers.
The move was a "sacrifice" that may need to be made to regain the public's trust in the exams system, he said.
His comments came in the wake of the Telegraph's December investigation, which also alleged that examiners had been secretly advising teachers on how to boost GCSE and A-level results.
The claims centred on teachers being given detailed advice at exams seminars on forthcoming exam questions and how students could score higher marks.
A Department for Education spokesman said: "It is vital confidence is restored in the exams system. We are committed to raising standards for all pupils.
"We are pleased Ofqual is taking action to ensure that GCSEs in these subjects are more challenging, requiring students to demonstrate that they have covered the whole curriculum.
"We want all exams in England to stand comparison with, and be as rigorous as, those in the best-performing education jurisdictions."