A court on Tuesday convicted two men of the 1993 murder of black teenager Stephen Lawrence, a landmark case that exposed racism in the London police and led to a change in the law allowing suspects to be tried twice for the same crime.

The 18-year-old school student was stabbed to death at a bus stop in southeast London in an unprovoked attack by a gang of white youths shouting racist abuse.

Gary Dobson, 36, and David Norris, 35, were found guilty after a six-week trial that hinged on new scientific evidence presented by prosecutors.

Lawrence's mother Doreen and father Neville wept as the verdicts were delivered at the Old Bailey, London's central criminal court, the Press Association reported.

Dobson protested his innocence as he was led from court saying: You have condemned an innocent man here, I hope you can live with yourselves.

The case became a catalyst for change after London's Metropolitan Police botched the initial investigation into Lawrence's death.

A 1999 report by senior judge William Macpherson said the murder had exposed institutional racism in the force and also accused officers of incompetence and a failure of leadership.

Since then, the police have overhauled their policies on racism and tried to recruit more officers from ethnic minorities, but the Lawrence case still weighs heavily on the force.

The impact of the Macpherson inquiry was felt across the public sector, with all bodies being required to put in place policies to prevent and address racism.

The Lawrence case also helped end the judicial doctrine of double jeopardy, which had previously prevented suspects from being tried twice for the same crime.

One of the defendants, Dobson, had been acquitted of the murder in 1996 when a private prosecution brought by the teenager's parents collapsed.

The Court of Appeal quashed that acquittal in May 2011 and said Dobson could stand trial again, a decision made possible after double jeopardy was scrapped in 2005.

The trial of Dobson and Norris, which began in November 2011, hinged on new forensic evidence linking the two men to the murdered teenager.

Prosecutors said textile fibres, blood and hair belonging to Lawrence had been found on clothing seized from the defendants.

The defence argued that the clothes were contaminated during the police investigation because officers did not store them properly.

(Reporting by Keith Weir and Estelle Shirbon)