Students are being forced to endure stress and tension which could be damaging their performance during AS Level exams, a study has found.

With the introduction of the new exam, the number of timetable clashes more than doubled, leading to complaints from headteachers and students alike.

In the wake of the criticism, England's exams watchdog has announced it is to hold a second consultation into the timetabling of AS level papers.

In his report, the former schools chief inspector Mike Tomlinson argued that pupils suffered from tiredness as a result of the change.

"The quality of work in AS papers, and in some cases the unevenness of performance across the paper, was noted," he said.

"Some awarders judged that the limited time available in year 12 [lower sixth form] to study for AS examinations could be the cause of some answers being limited in one or more ways.

"Others attributed this effect to result from the change to examining all AS units in a subject in one session."

"In the light of the uneven performance at AS, the QCA should reconsider its earlier decision to examine all AS units in a subject in one session," he advised.

The Liberal Democrats argued that students faced too many exams.

"When students are taking as many as 23 external exams within an 18-month period, the emphasis is going to be on testing rather than learning," said education spokesman Phil Willis.

"After a year of Mike Tomlinson working diligently and an examinations system under pressure, absolutely nothing has changed.

"Liberal Democrats believe that the majority of AS assessment should take place within schools, using chartered examiners, rather than putting young people under extreme pressure for little gain."