A "guardian of the truth" on Wikipedia, the global internet encyclopedia, has been caught up in an embarrassing scandal after it was revealed that he created bogus online identities to change entries on the system.

David Boothroyd – a London councillor by day, a cyber policeman by night – has been forced to resign from Wikipedia's Arbitration Committee after his alias editing gave rise to a major conflict of interest.

The Labour councillor's fall from grace comes two years after he fought off stiff international competition to win a coveted seat on the 15-strong committee, which is responsible for settling hundreds of editorial disputes every day.

His membership of "ArbCom" was no longer tenable after it emerged that he had committed one of the most serious crimes in cyberspace: sockpuppeting – the use of multiple online identities to create the illusion of support for a point of view, person or organisation.

A log of publicly available page edits exposes several changes to Tory leader David Cameron's Wikipedia entry by Mr Boothroyd under the alias of Sam Blacketer, including changing the picture to one "not carrying saintly overtones".

Mr Boothroyd claims the whole thing was an "innocent oversight" and points to the numerous favourable corrections he has made to Mr Cameron's page.

Paul Williams, volunteers director at Wikimedia UK, the British arm of the American company, said: "Sock-puppeting is a very serious offence for anybody. But for someone on the Arbitration Committee it is even more so. It can result in a lifetime ban. The problem with Wikipedia is that you can hide behind user names, but there is an expectation that you don't write for self-interest. In this case there is a conflict of interest."

The online encyclopedia has been plagued with disputes since its launch in 2001 by Jimmy Wales and Larry Sanger. Initially hailed as a utopian vision – an encyclopedia for the people, by the people – it quickly attracted criticisms about the inaccuracy of its content. But with 13 million entries, Wikipedia is the world's largest online encyclopedia and the eighth most popular site on the web.

The committee adjudicates on all matters. From the trivial (whether J K Rowling's surname is pronounced "ow" or "oll") to the political (the geographical borders of Palestine). It can also ban people from writing on the site, a punishment which has been meted out to Scientologists who have tried to erase criticisms of their creed.

Mr Boothroyd said: "I can be a partisan when I want to be, but I can look at things objectively. I created different user names because people found out I was a Labour councillor and kept harassing me, accusing me of being biased. Getting a new name was a form of defence. I have never written to self-serve but I admit it was wrong, although an innocent oversight."