More than half of teachers believe internet plagiarism is a serious problem among sixth-form students, a teaching union survey suggests.
Identifying plagiarism is time-consuming for teachers
The 58% of 278 teachers who identified it as a problem said they thought 25% of work returned by pupils included material copied from internet sites.
One teacher said a piece of work they saw still contained website adverts.
The Association of Teachers and Lecturers said schools must introduce robust policies to tackle plagiarism.
The survey was carried out in December among ATL union members who teach sixth-formers in schools and Further Education colleges in England, Wales and Northern Ireland.
Gill Bullen, a teacher from Itchen College in Southampton, told the survey: "Two GCSE English retake students were very late handing in their last piece of coursework, an essay on Romeo and Juliet.
"When finally given in, the pieces turned out to be identical - and significantly better than either of them could have done.
"Not only that, the essays given in didn't quite answer the title question I had set."
Another teacher from Leeds, who took part in the survey, said: "I had one piece of work so blatantly cut and pasted that it still contained adverts from the web page."
ATL general secretary, Dr Mary Bousted, said: "This survey highlights one of the risks of putting so much emphasis on passing tests and getting high scores at any cost.
"And teachers are struggling under a mountain of cut-and-pasting to spot whether work was the student's own or plagiarism.
"Schools and colleges need to have robust policies to combat plagiarism, but they also need help from the exam boards and government with resources and techniques for detecting cheating."
It takes up precious time to spot plagiarism, say teachers.
Mark Jones, from Wirral Metropolitan College, said: "Any work found to be plagiarised will not be marked - the student has to do it again.
"However, the problem is that, with the best will in the world, you haven't got enough hours in the day to search out where information was plagiarised from to prove it."
More than 55% of teachers questioned in the ATL survey said students did not have sufficient understanding of what plagiarism was and what counts as legitimate research.
Diana Baker, from Emmanuel College in Durham, said: "I have found once students clearly understand what plagiarism is, its consequences and how to reference correctly so they can draw on published works, plagiarism becomes less of a problem.
"I think the majority of students who engage in plagiarism do it more out of ignorance than the desire to cheat, they really want to succeed on their own merit."
Having a robust school or college policy on plagiarism seems to be critical.
However, more than 55% said either their school did not have a policy to deal with plagiarism or they were unaware of one.
Schools minister Jim Knight said: "Despite our rigorous system, more needs to be done to assure all parents that coursework assesses pupils' work in a fair and robust way.
"Last year, we asked the Qualifications and Curriculum Authority to look at making GCSE coursework more robust and reliable.
"As a result of the QCA's report, we will be removing all GCSE coursework from maths and stipulating that in other subjects, coursework must be supervised in classroom style conditions.
"We will work closely with teachers to develop even more effective and reliable coursework assessments."