The world's largest search engine, Google, is to ban adverts from companies that offer essay-writing services.

The internet firm has written to companies that sell essays and dissertations online informing them of the ban, which is due to come into force next month.

The move was immediately welcomed by universities, which are keen to root out student cheats.

Prof Drummond Bone, the president of Universities UK (UUK), which represents vice-chancellors, said: "Making life harder for these cynical web 'essay mills' is a step in the right direction."

He said plagiarism devalued the efforts of students who worked hard to achieve their degrees.

"Universities UK members have severe penalties for those students caught cheating, with many institutions already using advanced anti-plagiarism software to make sure that this is enforced," he added.

"Tackling plagiarism and cheating is, of course, an issue for individual higher education institutions. But the sector as a whole will surely welcome all attempts to help curb the growth of these websites."

However, the move has angered essay-writing companies, who believe the ban will harm their legitimate business.

Matthew Wilson, the managing director of Essaywriter.co.uk, told the BBC that the ban would punish legitimate companies that sell essays, but warn students not to use them dishonestly. The company charges at least £70 for an essay, but insists they should only be used as a guide for students.

Student plagiarism has become a serious problem in universities. Around 90% of UK institutions are believed to now use plagiarism detection computer software to catch cheats.

Last year, UUK held a conference on the issue to allow universities to share good practice to tackle the problem. The conference followed a survey of university students that found that more than half believed their lecturers would not spot work plagiarised from the internet.

One university has now drafted an "honour code" that students agree to follow, which includes a statement that they will avoid plagiarism. A similar idea is already operating in US universities.

Essay-writing companies join a growing list of content deemed inappropriate by Google. These include adverts for tobacco, prostitution and fake documents.