Teachers have signed off exam coursework as original even when it was obvious that pupils had colluded with each other, documents seen by The TES reveal.
Examiners' reports criticise lax checks in at least one high-profile course. In other cases, they comment on the striking similarities between some pupils' work.
In the latest coursework controversy, examiners also complain that teachers have been over-generous with their marking.
The findings come from a TES analysis of recent chief examiners' reports across a range of GCSE, A-level and GNVQ subjects.
One academic likened the willingness of some teachers to do all they could to improve coursework results to coaches condoning drug-taking among athletes.
Professor Alan Smithers, of Liverpool university, said: "Athletes may be tempted to take drugs, and pupils to use illegitimate means to improve their performance. Some coaches, and unfortunately some teachers, connive in this."
The most striking examiners' comments come in the latest available Edexcel report for GNVQ intermediate information and communication technology, the qualification equivalent to four GCSEs.
Teachers, who act as internal assessors for the qualification's coursework, should sign an "authentication statement", verifying that the work is the pupil's.
But in the 2002 report, the chief examiner wrote: "In some (schools and colleges), teachers either did not sign the authentication statement for all candidates or submitted an authentication statement with a photocopied signature.
"In some cases, teachers signed authentication statements for some candidates, when a cursory glance by the examiner indicated collusion."
The report added that in a few schools and colleges, "the work of all candidates was similar", suggesting that teachers had provided too much "guidance" to pupils. Edexcel's chief ICT moderator also reported problems with teachers' attempts to verify work.
Problems were reported by other boards' examiners.
The report on Edexcel's land and environment GNVQ for summer 2002 found evidence, for the second year running, of portfolios containing work often downloaded from the internet or from CD-Roms.
An AQA board report on this year's sociology A-level said: "In some candidates' work, there was evidence of copying large sections of the exemplars given to teachers at standardisation meetings."
AQA also reported problems on this summer's environmental science A-level:
"In the majority of centres, the marking was judged to be generous, and in some centres it was extremely generous."
The report on this summer's AQA GCSE history, praised the coursework standard, but added: "coursework needs to be the independent work of candidates".
Some schools will argue that pressures of time, rather than deliberate manipulation, lay behind the marking problems.
However, this is just the latest in a series of controversies to hit coursework, introduced with the launch of GCSEs in 1988.
Earlier this year, we reported how contributors to The TES website had claimed that schools were bending the rules, including allowing pupils to redraft their submissions.
Private tutors have told The TES that they are approached at least once a week by students asking them to write their GCSE and A-level assignments for them.
An Edexcel spokesman said that coursework was subject to strict checks by moderators. Work which was out of line with a pupil's performance in other aspects of an exam would be picked up.
He added: "We are satisfied that the processes ensure that the marks that are awarded are the true marks due to the student."
A QCA spokesman said: "We take any allegation of cheating very seriously."
Eamonn O'Kane, general secretary of the National Association of Schoolmasters Union of Women Teachers, said: "Maybe these examiners' comments will give people pause to consider again the balance of coursework against external exams. I think there should be less coursework."