History was more "demanding" than geography at GCSE and AS-level, while chemistry was the most difficult of the three sciences at all levels, the Qualifications and Curriculum Authority (QCA) said.

The watchdog said the subjects reviewed in the study were "broadly comparable" in their difficulty overall.

But there were concerns that pupils were given credit for "prepared" answers in some A-levels, while other students won marks for writing "uncritically" about their own lives.

The report compared a selection of groups of subjects and did not produce an overall ranking of all secondary school exams. Headteachers called for a full study to be undertaken.

Martin Ward, deputy general secretary of the Association of School and College Leaders, said: "Of course some people have an aptitude for one area over another. But school and college leaders know that exams in all subjects are not equal.

"Students choose a variety of subjects at both GCSE and A-level, and the perception remains very strong that some subjects are harder than others.

"This skews student choice and, because the harder subjects are the physical sciences, mathematics and modern foreign languages, this situation works to the disadvantage of the country and its future prosperity."

Schools Minister Jim Knight said comparing subjects was "complex". He added: "The level of demand is broadly comparable across the subjects considered in the studies."

QCA chief executive Ken Boston said: "Ensuring comparability between examinations, awarding bodies and across subjects is vital to maintaining standards."