A host of blue-chip companies have altered their entries on Wikipedia, the online encyclopaedia, in an attempt to cover up embarrassing episodes in their history.

The discovery was made by WikiScanner, a site that traces the source of changes to the world’s largest online reference work by matching edits - which, famously, can be made by anyone - to a database of the unique “IP addresses” of the computers that were used to make them.

Machines belonging to organisations including Wal-Mart, Disney, Sony, the Labour Party, the CIA and the Vatican, have been used to rewrite entries, it alleges. WikiScanner was developed by Virgil Griffith, 24, a researcher at the California Institute of Technology.

In one example, references to claims that Seroquel, a drug developed by AstraZeneca, carries a risk of making teenagers “more likely to think about harming or killing themselves” were deleted by a user of a computer shown to be registered to the drug company.

In May the US Food and Drug Administration proposed that makers of all antidepressants, including Seroquel, update labelling to include warnings over increased risks of suicidal thinking in young adults.

AstraZeneca said it was “investigating questions regarding a change to the Wikipedia entry”. It added: “IP addresses may not identify locations or affiliations of internet users with certainty - they can be falsified.”

Graham Cluley, of Sophos, the web security expert said: “It may be possible to fake an IP address but it’s far from trivial. You would probably have to remotely take over a company’s computer network.”

A computer registered to the Dow Chemical Company is recorded as deleting a passage on the Bhopal chemical disaster of 1984, which occurred at a plant operated by Union Carbide, now a wholly owned Dow subsidiary. The incident cost up to 20,000 lives.

A machine that belongs to Exxon-Mobil, the oil giant, is shown making sweeping changes to an entry on the Exxon Valdez oil spill of 1989. An allegation that the company “has not yet paid the $5 billion in spill damages it owes to the 32,000 Alaskan fishermen” was replaced with references to the funds the company has paid out.

Mr Griffith admits that WikiScanner cannot identify the individuals behind alterations. “However, we do know that the edit came from someone with access to a [company’s] network,” he said.

Wikipedia original text about Exxon

‘Mobil has not yet paid the $5 billion in spill damages it owes to the 32,000 Alaskan fisherman’

Wikipedia text about Exxon after alteration

‘Exxon Mobil paid $300 million immediately and voluntarily to more than 11,000 Alaskans and businesses affected by the Valdez spill’

Times Recommends

• Lies and cover-up at MG Rover exposed by report
• High oil prices push manufacturing costs up
• Primark founder steps down as chief executive