Teachers who allow coursework cheats to escape detection could be charged with misconduct, new guidelines say.
The QCA warned about internet plagiarism last year
The government's exam watchdog in England, the Qualifications and Curriculum Authority, has published a new coursework guide for teachers.
Last year the QCA published a report warning that internet plagiarism "could not be controlled", and the education secretary called for an urgent review.
The Department for Education said the new guide was "a welcome development".
Education Secretary Ruth Kelly asked the QCA to review coursework in every GCSE subject to try to tackle the problem of coursework cheating.
The QCA guidelines are to help teachers "minimise opportunities for malpractice".
They say teachers should familiarise themselves with sites which offer writing services so that they can more easily spot plagiarised work.
Ensuring they are familiar with each candidate's work and writing style will help them identify work which could have been written by somebody else.
They are also told to check for "inconsistencies" such as irregular font or margin size, and to compare carefully the standard of supervised and unsupervised work.
Where teachers have suspicions, they should ask the candidate oral questions or get them to acknowledge their sources, before considering reporting their concerns to the exam board.
"If they are unable to answer your questions and display a lack of understanding of the issues involved in the coursework, it could indicate that the work is not their own," the guidelines say.
They also advise teachers to "highlight the deficiencies of online essay banks" and "provide pupils with practical examples of good and bad referencing".
Candidates must sign a declaration saying that the work they are submitting is their own, and teachers must also declare they are confident the coursework is the child's own before submitting each piece to the moderator.
Last year's QCA report said many pupils were aware of sites offering essays online, and some had admitted trying to download them.
And it warned that some parents and teachers were effectively doing the coursework for their children. Almost two thirds of parents said they gave at least some help.
But the report also emphasised the value of coursework in providing opportunities for in-depth study and giving pupils responsibility for their own learning.
It also expressed concerns about "cloned coursework" and "scaffolding" answers - where so much help was given by teachers in the form of templates, checklists and framing essays, that there was little to tell candidates' work apart.
But the new guidelines make no mention of how much of this type of help teachers should be able to give.
A spokeswoman for the Department for Education and Skills said: "Coursework must be rigorous and the regulator has a role in ensuring that the rules and regulations in relation to coursework are clearly defined and understood by teachers, students and their parents."
The results of the wider review will be published this spring.