Examiners believe that at least a fifth of exam grades awarded to pupils are incorrect, research conducted for Ofqual, the qualifications regulator, has found.
The Ipsos Mori study asked a representative focus group of examiners whether they had concerns about the amount of error in the system and found they believed "at least 10 per cent of grades are incorrect".
"But that figure only takes into account those whose grade is lower than they deserve," the report says.
"They feel that an equal number may be receiving grades higher than they really deserve (something people don't really like to talk about!)."
The study consulted teachers, pupils, parents, employers and members of the public, as well as examiners, and found that all groups questioned the logic of having multiple exam boards.
Despite examiners "readily" admitting that mistakes were made, the study found that they felt "the measures in place to reduce examiner-related error are effective".
The belief that at least 20 per cent of grades are incorrect emerged from a focus group of four examiners.
An Ofqual spokesperson said the examiners' opinions were "just one example of how those within the system perceive variability in marking of examinations".
The study follows a Qualifications and Curriculum Authority (QCA) report in March that suggested only 45 per cent of pupils who sat the key stage 3 English writing test in 2007 received the correct grade.
The new research found a "general consensus" that exam marking needed to be taken more seriously.
"People are quite surprised to learn that many examiners are actually teachers who mark papers in their spare time," the report says.
"They question whether this set-up in itself contributes to error, and argue that the level of examiner-related error in the system may be easier to control if teachers who examine take time out ... to be properly trained, and if there are some professional examiners for whom examining is their main career."
The study also found that examiners thought schools chose the board they believed would get their pupils the highest marks to help them climb league tables, and that "boards play up to this by adapting their products accordingly".
Parents, pupils and the public all had concerns about the number of exam boards.
"There is bemusement over why one body sets the curriculum, only for several separate bodies to then design their own questions," the report says. "No wonder, they say, there is room for error to creep in."
Teachers agreed that the system "lends itself to errors" and felt it would be clearer if everything was controlled by the QCA. Employers did not recognise "any benefit" in having more than one board.