LONDON (Reuters) - Britain's student cheats and their exasperated lecturers are trying to outwit each other by increasingly sophisticated means.

As many as 25 percent of university students copy and paste material from the Internet and present it as their own work, the first national study into plagiarism has found.

Not that the practice is confined to academia -- before the Iraq war, chunks of an infamous government dossier making the case for invasion were found to have been lifted from work by an American student.

Now universities have pledged to get tough on copying by running students' work through detection software that compares it to 4.5 billion Web pages and produces an originality report.

But the software will not catch the increasing number of students who order a custom-written essay online.

Fiona Duggan, manager of the government-funded Plagiarism Advisory Service, said plagiarism had mushroomed over the last few years.

"When access to the Internet became widespread, suddenly there was all this information out there and you were faced with the prospect that your students would be accessing information which you had no idea existed," she told Reuters.

"Before that in most cases a lecturer would be an expert in their subject area and would know the key sources and, in lot of cases, have written the key books."

Hundreds of online essay banks and research sites offer ready-made top-grade answers for the public examinations taken at age 16, 18 and degree level in Britain.

But whereas a teacher who has tracked a pupil's progress is more likely to spot a suspicious piece of work, university lecturers who have large numbers of students and little individual contact may not.

Yet some cheats can't help but give themselves away.

One academic came across: "please click here for more information" in the body of a student essay. Other students have forgotten to delete the: "thank you for buying your essay from ..." at the bottom of their work.

Academic Answers Ltd. runs five Web sites which provide essays to a customer's specifications, including the grade they require.

An upper 2nd (2.1) grade standard degree dissertation costs 29 pounds per 500 words. For next day delivery the rate rises to 59 pounds.

Academic Answers sales manager Jonathan Cooke said the company did not condone plagiarism and did all it could discourage clients from passing work off as their own -- even blacklisting those who phone up saying: "I need a piece to hand in urgently."

"Our customers vary from the one who is really struggling with a piece -- who doesn't know where to begin or what textbooks to read -- to others who constantly come back for an insight into the work they are completing," said Cooke.

The essays aid research and show students the kind of standard to aim for, he added.

The advisory service's Duggan believes cheating can not simply be stamped out overnight: a wider approach is needed including teaching students what constitutes plagiarism and looking at universities' assessment structures.