A glitch at Google in which every site on the internet was labeled as potentially harmful to users' computers has been blamed on human error.

Surfers across the world complained as search results came back with the ominous warning 'this site may harm your computer' for every single web site on 31 January at around 15.00 GMT.

The problem, which lasted for about 40 minutes, was initially blamed on bad data from StopBadware.org, a non-profit group run by Harvard and Oxford universities. But Maxim Weinstein, leader of the StopBadware team, quickly denied that his organisation was to blame.

"Google generates its own list of 'badware' URLs, and no data that we generate is supposed to affect the warnings in Google's search listings," he said.

Google afterwards admitted that the mistake was down to an error by its own staff. An employee in charge of compiling the search engine's list of dangerous sites mistakenly entered a forward slash (/) character as a value to the file, extending Google's blacklist to every single site on the internet.

"This was clearly an error, and we are very sorry for the inconvenience caused to our users," wrote Marissa Mayer, vice president of Google's Search Products & User Experience, in a company blog post.

Mayer went on to say that Google will "carefully investigate this incident and put more robust file checks in place to prevent it happening again".