A new website built by an American technology student has uncovered the lengths that companies apparently go to improve their public image by tweaking their entries on Wikipedia, the online encyclopaedia that - famously - “anyone can edit”.

The WikiScanner site, developed by Virgil Griffith, a researcher at the California Institute of Technology, reveals changes to the online encyclopaedia by linking edits back to the computers from which they emanate using each computer’s unique IP address.

Mr Griffith, 24, says he created the site "to create minor public relations disasters for companies and organisations I dislike" - a mission he may well have succeeded in.

Among those he alleges have been updating their entries are Wal-Mart, the world’s largest grocer, AstraZeneca, the drugs giant, Britain's Labour Party, the CIA and the Vatican.

In one example he gives, a computer linked to an IP address registered to the Dow Chemical company is seen to have deleted a passage on the Bhopal chemical disaster of 1984, which occurred at a plant operated by Union Carbide, now a wholly-owned Dow subsidiary.

WikiScanner cannot identify the individuals altering Wikipedia articles. It can show only that an edit was made by a person with access to an organisation’s network.

"Technically, we don't know whether it came from an agent of that company, however, we do know that edit came from someone with access to their network," Mr Griffith says on his site.

A slew of other companies' computers are also shown to have been used to polish Wikipedia entries.

ExxonMobil, the US oil giant, made sweeping changes to an entry on the Exxon Valdez oil spill of 1989. A claim that the company “has not yet paid the $5 billion in spill damages it owes to the 32,000 Alaskan fishermen” is deleted and replaced with references to the funds the company has paid out.

A web surfer using a machine on Wal-Mart’s network has amended a passage on the rates that the retailer pays its employees - to the benefit of the world's largest retailer.

A computer registered to Disney, the media giant, was used to delete a reference to criticism of the use of Digital Rights Management software, used by the group to safeguard digital media from piracy.

According to other Wikipedia pages laid bare by the Wikiscanner site, references to claims that Seroquel, a drug developed by AstraZeneca, which allegedly made teenagers “more likely to think about harming or killing themselves” were deleted by a user of a computer registered to the drug company.

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