Most teachers show faith in the exam system, but a third have worries over marking accuracy.
Confidence in A-levels is at a five-year high, despite around one in three teachers believing the quality of marking has deteriorated in the past two years.
In a survey by the exams regulator, out of 31 per cent who said they did not have confidence in the accuracy of A-level results, most cited a lack of experienced examiners.
The findings in the Qualifications and Curriculum Authority's annual report come despite exam boards' heavy investment in marking technology in recent years.
Last week, The TES revealed how the authority has launched an inquiry into marking quality, with Eton College among schools unhappy about strange results. Two of its pupils received zero marks in a Russian A-level but had them changed to A grades after a re-mark.
A total of 59 per cent of A-level teachers surveyed for the authority by Ipsos Mori said they had confidence in A-level marking, with 10 per cent undecided. Of the 31 per cent who disagreed, a quarter mentioned a lack of good examiners, with 22 per cent agreeing with the statement "numerous requests for re-marks have shown up errors".
At GCSE, 23 per cent lacked confidence in the accuracy of marking, with 67 per cent saying they were confident. Some 31 per cent said marking quality had deteriorated in the past two years.
A third of teachers said they could only be sure that GCSE and A-level papers had been marked accurately after requesting checks or re-marks from the boards.
"Teachers ... feel this to be necessary to ensure the students get the results they deserve," said the survey of 500 teachers.
But such challenges are used in only a minority of cases. At A-level, four out of 10 said most students got the results they deserved.
And support for the A-level has risen to its highest point since 2003: 76 per cent of teachers said they generally had confidence in the system. A similar proportion said the same about GCSEs.
In 2003, after a grading furore sparked by independent schools the previous year, only 54 per cent of teachers said they had confidence in A-levels.
Mary Bousted, general secretary of the Association of Teachers and Lecturers, said: "The English exam system is one of the most professional in the world, but we are becoming increasingly concerned about standards of marking. It's time to rethink assessment along with the national curriculum, and give a bigger say to the teachers."
However, the Joint Council for Qualifications, an umbrella body for exam boards, said this week that very few grades were changed as a result of challenges from schools.
Andy Schofield, head of Varndean School in Brighton, East Sussex, said: "The marking of GCSEs and A-levels does seem in general to be a little bit looser than it was in previous years, but we have not had any particular issues with it."
Most teachers do not believe A-levels are threatened by the Government's introduction this year of new work-related diplomas. Only 11 per believed the 14-19 diplomas would undermine confidence in the "gold standard" exam, while 8 per cent said they believed they would strengthen A-levels.
Some 44 per cent said the introduction of the diplomas would make no difference to A-levels.
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Plot thickens in tale of weird results
Teachers have been sharing "horror stories" about strange marking patterns at the hands of various boards, The TES has been told.
An English teacher from a secondary in the Midlands said training days to prepare teachers for new A-level syllabuses last term were replete with complaints.
The teacher, who asked not to be named, said his school had suffered from erratic marking for years. In one board's English literature A-level in 2006, four of his pupils scored 120 marks out of 120 in one paper, then managed only three Cs and a D in the next, which was taken straight afterwards.
Some 42 per cent of papers sent for a re-mark went up a grade. The teacher said he was happy with the English language A-level grading, but less so with GCSEs. He was worried the introduction of the A* grade at A-level in 2010 would be blighted by unreliable marking.
He added: "As a state school, we cannot afford to pay for every injustice, nor can we advise parents to spend money on a re-marking system that's arbitrary in its results."
Fiona Kisby, a former teacher at a Hertfordshire independent school, said a third of her students gained 100 per cent in at least two of their six A-level history papers last year, but either failed or just scraped passes in one paper, a project. She said: "I believe the perfect scores are as meaningless as the failures."