More than a quarter of A-level papers received the top grade this year, the highest percentage ever, according to figures released on Thursday.

The number of A-grades rose by 1.2 percentage points to 25.3 percent, the Joint Council for Qualifications said.

A-grade passes have risen by almost five points in the last five years, reigniting the long-running row over whether exams have been made easier to improve pass rates.

Dr Mike Cresswell, director general of the Assessment and Qualifications Alliance, the largest of the three exam boards, denied accusations of "dumbing down".

He said the rate of improvement in A-grade passes was up around 6 percent at independent schools but there had been almost no improvement in the number of A-grades being awarded to pupils at secondary modern schools over the past five years.

"There would simply not be differences like this in the results if the exams were just getting easier every year," he added.

"They reflect the hard work and skills of teachers. They reflect the motivation of students."

The overall cumulative percentage of candidates who passed with an A-E grade rose 0.3 points to 96.9 percent.

Girls continue to outperform boys at every major subject, apart from modern foreign languages and further maths.

Traditional subjects such as biology, chemistry, English, geography, history and maths continue to be the most popular.

English and maths are the most popular choices.

The National Union of Students said results have improved because pupils and teachers are working harder and not because tests have been "dumbed down" to boost pass rates.

"We seem incapable of celebrating the educational achievements of students," said NUS Vice President Beth Walker.

The University and College Union, which represents 120,000 academics and other staff, said pupils should ignore the "doomsayers sniping from the sidelines with their annual rant about falling standards".

"Today should be about celebrating the achievements and hard work of students and teachers," said UCU chief Sally Hunt.

A study published this month suggested that the 55-year-old "gold standard" exam really is easier.

Researchers at Durham University said average results for pupils of the same ability had improved by two grades between 1988 and 2006.

An Institute of Directors survey of business leaders this month found that more than half thought education and skills had not improved since 1997.