Michael Gove’s A-level reforms will result in a more inflexible system that is “unforgiving in a Christian country”, an independent school leader has claimed.

Plans to scrap the current system of AS levels could also lead to underachievement as young people have less room for error when making their A-level choices, said Alice Phillips, the new president of the Girls’ Schools Association.

The current system, due to be phased out from next year, successfully broadened the curriculum while maintaining depth and giving pupils flexibility, she said.

Ms Phillips, head of St Catherine’s School, a girls’ private school near Guildford, Surrey said: “When they brought in the current system [including AS levels], they removed at a stroke the bad old days of people … doing three subjects, suddenly discovering one was the wrong subject and finding that they couldn’t change.”

She said that before the AS system, pupils found themselves “absolutely trapped” if they made the wrong A-level choices, performed poorly in a subject they were not suited to and went to a “lesser university” as a result.

Under the AS system, students typically study four or five subjects for one year and narrow that down to three or four in the second year.

“I feel very strongly that at 16 or 17 there should be some accommodation for a mistaken path or a year in which you don’t perhaps work your hardest,” Ms Phillips said.

“The whole point about AS is that it gave you the flexibility at the end of the year. You could lose one and still have three; you could just choose more freely, mix and match things … keep the options open a bit longer.

“Not to be able to have one more chance if you’ve been a bit idle or unfocused, seduced by the freedoms of the sixth form, not to have the chance for a call to arms is pretty unforgiving in a, we call ourselves, a Christian country.

“There were an awful lot of people under the old regime who had very little to show for it at the end of two years. There were people who went through school and came out with a straight run of failures.”

Ms Phillips said that she understood the need for the system needed to be “tweaked”, but a complete overhaul was expensive and unnecessary.

Her comments chime with other educationalists who have questioned the return to linear A levels - due to start coming into effect from September 2015.

The University of Cambridge has already said that ditching AS levels will mean they will have to put too much emphasis on GCSE grades, predicted A-level grades and interviews in admissions - which they claim are less reliable.

Admissions tutors at the University of Oxford have said that AS levels give students a snapshot of what they are capable of achieving in the middle of their courses, spurring them on to apply to university when they might not have done so.

Under the new regime, A levels will take the form of end-of-course exams, and modules and repeated retakes will be scrapped. Existing AS levels will be retained as stand-alone courses but results will no longer count towards the full A level.

Ministers say that the switch will free up teaching time and prepare students for the demands of higher education.

Meanwhile, Ms Phillips also claimed that mistakes were being made in exam marking because tired teachers were being forced to grade papers at night after doing a full day’s work.

A move towards electronic marking - where makers have to use a designated computer outside of school - has made it difficult for examiners to mark scripts during school hours when they are “at the height of their game”, she said.