Sixth-formers attempting to cheat by buying essays online to submit as their own risk being sold poorly written, substandard work, the exams regulator has claimed.
Ofqual warned students against using such services, suggesting that much of the writing provided is not up to scratch and is unlikely to achieve decent grades.
A study published by the regulator which assessed a number of essays from online firms found many of those analysed were littered with errors and of poor quality.
Chief regulator Glenys Stacey said young people should be aware of the consequences of using these services, and she warned that anyone who buys essays online is not getting value for money.
Those caught cheating are at risk of losing their whole qualification, Ofqual said.
The "mystery shopper" study, conducted by London Economics, looked at three companies offering essay and coursework help - Custom Essays (customessays.co.uk), UK Essay Writing Services (essayservice.co.uk) and UK Essays (UKEssays.com).
Each was commissioned to provide a grade A standard 2,000-word A-level history paper on the short-term significance of the National Insurance Act of 1911, and a grade A standard 1,250-word A-level English Language essay on Nelson Mandela's death and the media, and to deliver the work within seven days.
The firms indicated they took on hundreds of essays a month in these subject areas, the report said.
Fees charged for the history paper ranged between £120-£220 and £70-£138 for the English essay.
But examiners' assessments of the six essays delivered showed they fell far short of top grades - with the history papers awarded B-E grades and the English work ranging between C/D to U/E.
The Custom Essays history paper was considered by examiners for the research to be of a B grade standard, while its English language essay was considered to have the lowest grade or all six essays analysed at U/E, the report claimed.
The study concluded that although in some cases the essays assessed were reasonably well constructed, there was an "almost universal ignorance" of the scope of the work needed and the criteria for assessment.
"Overall, the commissioned essays were considered to be of poor quality and fell well short of what might be expected from a representative student at this stage of their academic career," it said.
In the case of the Custom Essays English essay, an examiner said it was "reading like the work of an unengaged, untutored floundering student" and "wholly descriptive and poorly written".
And one examiner said that part of the Custom Essays history work "does not sound like an 18-year-old unless they are rather pompous".
One examiner assessing the UK Essays English paper said: "If this were the work of a student then we'd have to conclude that she/he was very articulate and very poorly prepared for this coursework task - relying largely on general comprehension and received understandings."
Ms Stacey said: "My message to students is quite clear. These essays are poor quality. Anyone who buys them isn't getting value for money. And more importantly, while there can be valid reasons for students buying these essays, such as essay practice or research, any attempt to pass this work off as the student's own is cheating.
"We want a level playing field for students, with each student assessed fairly, and so decided to look into this issue more closely. One of the more shocking findings was that the essays supplied by these companies are not even the grade they claim to be. For example, essays that claimed to be grade A quality were purchased and when they were analysed by our experts, the majority were only a C or even a grade D.
"Teachers are also advised to look out for these essays being submitted. They can look out for a change in style in how the student normally writes, or even a very fast turnaround of a piece of work. Exam boards should also be doing everything they can to stop these essays being submitted into the system and they have a responsibility to ensure their malpractice monitoring systems are effective."
Tony Eynon, chief executive of All Answers Ltd, which runs UKEssays.com said: "We are extremely disheartened and disappointed by the wholly inaccurate and presumptuous report published by Ofqual, and have no option but to refute a number of their conclusions.
"Of particular note is a heavily biased focus against the student support industry and a narrow look at this segment of the market.
"The methodology of the paper is deeply flawed and inconsistent, using a prejudiced approach in marking the essays.
"Papers were not marked anonymously, and examiners were informed about the source of the essays prior to marking.
"There were also a number of incorrect assumptions and misinterpretations of our sales procedure."