Google has announced that it will block adverts for essay-writing services. Google is to ban adverts for essays - which are sold to students for £5 to £5,000 via the search-engine - over fears that plagiarism is damaging the integrity of UK university degrees.

From June Google will no longer host adverts from companies which sell essays and dissertations. Google has contacted essay-writing advertisers to tell inform the of its decision.

Google has a blacklist of types of products and services for which it will not accept advertising, including firearms, tobacco, fake documents and prostitution.

Google said its advertising policies are "developed and evaluated based on multiple factors, including legal and cultural considerations". Google said the essay-writing advert ban would be applied across Google's global network.

Universities feel that plagiarism via Google devalues university degrees. Many colleges have publicly denounced the habit of students buying customised essays over the internet.

As a consequence, higher-education authorities have warmly welcomed Google's action against essay-writing services. But essay-writing firms say Google is unfairly punishing their legitimate businesses.

A Universities UK spokesman said that the biggest problem was with companies which sell tailor-made answers. Such essays are often penned by academics and it is less easy for plagiarism software to detect such work. Universities UK rejected the claim that students would pay hundreds of pounds for model answers and then not submit them as their own work.

But, one of the companies affected by Google's ban, said 80 percent of their customers coming through Google. Managing director Matthew Wilson told the BBC that the ban will punish legitimate companies that sell essays, but warn students not to use them dishonestly.