Sixth-formers who get all the questions right in A-level maths could still fail to reach 100 per cent because of puzzling new rules from the Government’s official exams and curriculum quango.

A marking system, known to officials as the “diving tariff”, has been condemned by angry mathematicians who say the effect is to punish students for taking mechanics, a subject crucial to British industrial success.

Mechanics has suffered for the past 20 years as sixth-formers, girls in particular, opt for the rapidly expanding field of statistics. The lack of interest in mechanics is associated with the declining numbers taking physics at A-level and engineering at university.

Now the School Curriculum and Assessment Authority has produced a mark scheme such that anyone choosing to add mechanics to the popular combination of pure maths and statistics will be entitled to less than full marks.

The mixture of all three is favoured by university maths departments. But students choosing it can only get 560 marks. The other mixtures - purestatistics and puremechanics - will be allowed a full 600.

“It is wholly untenable to offer students just 93.33 per cent of their final marks,” said Ann Kitchen, the newly appointed chair of the Association of Teachers of Mathematics.

“If there had been more consultation, this problem would have been ironed out long ago.”

“The mark implications are really quite extreme,” said Roger Porkess from the Mathematics in Education and Industry syllabus group. MEI maths, a six-module A-level that is taken by roughly a sixth of students, is particularly affected.

As a result, he said, a candidate scoring As in five papers and achieving the top mark in the sixth, mechanics, paper could still fail to get an overall A grade.

“That’s a really severe penalty,” said Mr Porkess.

Peter Swindlehurst, a senior executive with the Engineering Council, said that both statistics and mechanics were important for understanding modern engineering.

“There’s a great use of statistics in modern processes: stress analysis and the probability of failure, for example. An understanding of statistics is an important part of engineering-based science education. University departments want sixth-formers to have done a good, broadly based maths paper.”

Dr Pamela Robinson, of Brunel University’s Centre for Education and Employment Studies, described the mark scheme as “a wholly unnecessary complication” in a subject already under great pressure.

The strangely named scheme of “diving tariffs” - after the scoring system for Olympic diving competitions - will make more marks available for some exam papers than others. The harder the paper (or dive) the candidate attempts, the more marks are available.

Students attempting to do a purestatistics A-level must do basic, easier, papers in both. But anyone doing a purestatistics mechanics exam will have to do basic papers in all three so will have fewer marks available.

A spokesman from SCAA said the proposal was still being considered and candidates would still have “access to the full range of A-level grades”. Syllabus writers, however, are under pressure to finish the new ASA-levels by the end of this month to meet deadlines.