A third night of rioting in London has raised suspicions of an orchestrated plan, as violent disturbances broke out sporadically across parts of the capital.

London police launched an investigation into the city's worst rioting in years as violence sparked by a fatal police shooting continued.

Scotland Yard said ''copycat'' looting had spread to several boroughs in the capital's north, east and south, while a mob of about 50 youths damaged property in Oxford Circus, at the heart of the city's tourist area.

London has had student and trade union protests turn ugly in the past 12 months but this outbreak of rioting was the worst for years away from the capital's centre.

Doubts have emerged over whether Mark Duggan, whose death at the hands of police sparked the weekend riots, was killed during an exchange of fire.

The Guardian understands that initial ballistics tests on a bullet, found lodged in a police radio worn by an officer during Thursday's incident, suggested it was police issue - and therefore had not been fired by Duggan.