The board said that, in the handful of cases it investigated in the past two years, the majority of papers had turned out to be fakes.
They added that pupils were being duped into paying up to pound;400 for the documents because they were desperate for a good grade. Supposedly leaked examination papers have been found on websites such as eBay, and for sale on the black market.
"The tragedy is some of these kids would have got a perfectly good mark anyway," said Jerry Jarvis, the board's managing director. The warning came as Edexcel announced a range of high-tech measures to discourage pupils from cheating which it will test in schools this year.
They include: ID tags, similar to those on CDs, which will track stolen exam papers; humidity detectors, so that the board can tell when sealed packets of exams have been opened too early; and mini-safes with built-in time locks, which will prevent papers from being accessed before the exam.
It is not yet known how many schools will trial the gadgets. Mr Jarvis said: "We have thousands of deliveries involving multiple drops to different locations and a breach of security has the potential to be very damaging.
"Occasionally we have thefts from schools, where papers have been left unattended." One breach in 2006 which involved a leaked A-level maths paper resulted in a student being barred from exams for four years and others being disqualified or having marks deducted. When papers are lost or missing the board is obliged to reissue the exam again.
"It's a logistical nightmare and it's liable to cause chaos in schools as well as potentially derailing students," Mr Jarvis said. "With the internet, that exam could be online in seconds."
Edexcel investigated 70 suspicious incidents in 2006, with one resulting in disciplinary action.