Twitter didn't cause the rapid spread of riots in London earlier this month, according to indications from a new study of 2.5 million tweets.

The study was prompted by concern over David Cameron's threat to close down social media sites in the event of future unrest. Home secretary Theresa May is due to meet representatives of Facebook, Twitter and BlackBerry make Research in Motion to discuss possible censorship of their services in response to future disturbances.

Preliminary analysis has been carried out by the Guardian newspaper on more than 2.5 million messages sent on the micro-blogging platform during the riots. A closer look at the timing of tweets reveals that Twitter used mainly to react and comment to the disturbances, rather than to organise looting and other criminal activity. The latter claim that has been levelled against Twitter, along with Facebook and the BlackBerry Messenger network.

The database of tweets covers the entire period of the disorder, which began on 6th August after armed officers of the Metropolitan police shot dead 29-year-old Mark Duggan in Tottenham, north London.

The Metropolitan police have since revealed that they considered ordering social networks to be switched off in a bid to quell the disorder, but were warned off by lawyers.

Hefty sentences have been imposed on rioters who posted plans on Facebook with the police vowing to track down Twitterers who participated in the disorder.

On a more cheerful note, the Guardian's research also revealed that more than 206,000 tweets (eight per cent of the total) related to clean-up efforts after the rioting.