One Edexcel employee said staff were astounded to find themselves marking extended writing papers from the religious studies exam - even though they had not studied the subject at GCSE.
"I thought examiners were supposed to be qualified," said the marker. "I really couldn't believe it."
The revelations came as pupils, teachers and ministers celebrated the largest improvement in headline GCSE results since 1992. The proportion of entries awarded A* to C grades rose from 59.2 in 2004 to 61.2 per cent. In Wales, they rose 0.6 per cent to 61.3 per cent.
But the gains are much less dramatic than those registered by the same pupils in England when they sat their key stage 2 tests five years earlier.
In 2000, the proportion achieving level 4 in English rose from 71 to 75 per cent; in maths from 69 to 72 per cent; and in science, from 78 to 85 per cent (see page 9). Yet this week's results saw only marginal improvements in GCSE English, English literature and maths.
Meanwhile, languages faced a fresh crisis after entries for French and German fell to their lowest levels since the early 1990s. In Wales, they were down more than 6 and 8 per cent respectively, and Welsh second-language entries fell by nearly a fifth (see page 2).
Most GCSE entries in Wales were with the WJEC, the Welsh exam board, but nearly a quarter were with English boards.
Earlier this week, Edexcel attacked newspaper reports about the use of its administrative employees as markers. Jerry Jarvis, its managing director, said everyone involved in the board's marking had a relevant degree or had worked as a teacher.
He said that non-teacher markers only "mark short answers, freeing up experienced examiners to mark the longer, reasoned answers and essays".
But the marker who spoke to The TES did not have a religious studies or relevant humanities degree and had never taught in a school. The GCSEs that staff marked included extended writing papers where candidates had half an hour to complete an essay.
Edexcel staff were offered pound;25 per hour for marking on week-days, and pound;50 at weekends - three times the rate for teacher-markers. Training varied from an afternoon's induction to a 20-minute talk through the marking scheme.
The markers used the board's pioneering computerised system, viewing scanned-in papers on-screen and then typing in points for each answer.
"The focus seemed to be all on the quantity of exams getting marked, not on the quality," the marker said.
Internal emails show the board was urgently trying to find employees to mark GCSEs in English, geography, business studies, history and religious studies as well as A-levels in French, geography, economics and general studies.
Edexcel confirmed some markers could receive just 20 minutes' training, but said this would only be when they were marking a single question.
"We do not allow people to mark if they are not up to scratch," a spokesman said. "We are confident in the process and in the quality of our marking."
NEWS2, LEADER 14