Pupils hoping to appeal against the results of their exams this summer will find it harder to get marks changed following a crackdown announced by the exam regulator.

Ofqual will toughen its stance on appeals to prevent some students getting a "second bite of the cherry" if they are unsatisfied with their initial mark at GCSE and A-level.

It follows an increase in the number of reviews requested by pupils and schools in 2015, with some 460,000 challenges lodged last summer.

Under the new system, a change to a mark will only occur if it is found a "clear marking error" has been made.

It is claimed this will lead to a "fairer" system, after it was discovered that some original marks were being overturned despite being "perfectly appropriate", meaning those who did not appeal were "unfairly disadvantaged".

But unions responded with scepticism to the announcement, with the Headmasters' and Headmistresses' Conference (HMC) saying in a joint statement with the National Association of Head Teachers (NAHT) it made the system "less clear, less consistent and less fair".

Ofqual's executive director for general qualifications Julie Swan said: "It is not fair to allow some students to have a second bite of the cherry by giving them a higher mark on review, when the first mark was perfectly appropriate.

"This undermines the hard work and professionalism of markers, most of whom are teachers themselves.

"These changes will mean a level playing field for all students and help to improve public confidence in the marking system."

The decision follows a consultation with schools' groups, subject associations, teachers and students.

More than 90,000 grade changes took place as a result of challenges lodged at GCSE, AS and A-level in the summer of 2015.

Of this total, only 639 qualifications shifted more than two grade boundaries.

In recent years there has been a marked rise in the number of reviews requested, growing from around 190,000 challenges in 2010 to more than 414,000 in 2014.

Independent schools were found to challenge more exam results than other types of schools and colleges.

Peter Hamilton, chairman of HMC's academic policy committee, said: "The changes Ofqual have announced today will make the proper review when marks and grades are challenged by schools and colleges less clear, less consistent and less fair.

"We are especially concerned that the proposals do not commit the regulator to ensuring that each candidate passing through the exam system has their papers marked and graded with the necessary high levels of consistency and precision. Without this, all claims about fairness ring very hollow."

The new guidance says that only "unreasonable" marks should be changed, a move NAHT general secretary Russell Hobby criticised for making "things more vague than they already are".

He added: " We agree that the current system has arbitrary features and this is why, with HMC, we have called for Ofqual to implement the method of re-marking that its own research shows is the most reliable and fair. We regret that they have not implemented their own research findings."

Other changes announced by the regulator include reviewers being monitored for consistency and the way being opened for exam boards to allow students to appeal to them directly, rather than through their school.

Association of School and College Leaders (ASCL) spokeswoman Suzanne O'Farrell said: "There remains an underlying problem in that the system is still something of a cottage industry in which teachers mark huge numbers of scripts in their spare time for little pay. It needs to be better resourced and better structured. The most important thing is to have a system that gets marks right first time."