Universities are introducing policies to limit students' access to plagiarism detection software amid concerns that some of them are abusing the system to cover up their copying.

It is common practice for tutors to encourage students to submit their work to programs such as Turnitin, to make them aware that their work is being scrutinised for plagiarism and to help them understand the rules of citation.

But debate is growing over whether students should be allowed to submit drafts of their coursework to such systems more than once. Some academics argue that doing so encourages students to attempt to "beat the system" by replacing words or phrases from copied text with synonyms and resubmitting their work until the software no longer detects the plagiarism.

Last summer, the senate of Cranfield University voted to place restrictions on students' use of Turnitin. It brought in regulations stating that "with the exception of a formative assessment at the start of a course, students should not be permitted to make multiple submissions to Turnitin UK when submitting work".

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This week, academics from the university will present a paper appealing against the restrictions at the Third International Plagiarism Conference at Northumbria University, organised by the Internet Plagiarism Advisory Service.

The paper, written by David Wright, a lecturer in the Centre for Systems Engineering, with Aurelie Owens and Nigel Donald, says: "A policy change of this magnitude was worrying since there was no evidence to support the suggestion that Turnitin had been subverted in this way. Not only did it limit the value of Turnitin as a formative tool but it also appeared to contradict the university's obligation to instil a sense of personal responsibility and academic integrity in its students."

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The authors argue that "it is better to devise a strategy to embrace the system's strengths and mitigate its weaknesses rather than fixate on a single shortcoming and significantly limit the usefulness of a potentially valuable tool as a result".

Stephen Bostock, head of the Learning Development Unit at Keele University, said that his university had encountered instances of students trying to subvert the system.

"We have had one or two cases where students had open-ended access and did seem to be progressively resubmitting work to get rid of the 'coloured' bits. It becomes a bit of a game, but it is not a very educationally useful one," he said.

"The advice I give to our academic conduct officers is that they should let students use Turnitin once and see their own reports, but not give them open-ended access.

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"We don't want to use Turnitin as a secret weapon to catch people out, we want to use it to deter any thoughts of either deliberate or inadvertent plagiarism."

Dr Bostock said the most important thing was that students got to use the system once. "I haven't seen any evidence - and I'm rather doubtful that you could get any evidence - that allowing them to use it repeatedly, certainly on the same piece of work, is going to get you any educational benefits. And there certainly is a risk that some students might try to beat the system."

Jude Carroll, an expert in plagiarism at the Oxford Centre for Staff and Learning Development, Oxford Brookes University, said: "People imagine students submitting, fixing, submitting, fixing. But I think it is a fantasy that tutors would let students have unbridled use. Most have a tutor-controlled mechanism - for example, they can do it just once within closely controlled times, or there is a 24-hour delay before they can resubmit."

rebecca.attwood@tsleducation.com.