By admitting that the man shot dead at Stockwell Tube yesterday was not one of the suspects in the July 7 attack or the attempted bombing on Thursday, the police have taken a potentially dangerous turning.

While officers in the US and other countries often shoot criminal and terrorist suspects, such incidents are rare in Britain, where only specialised units are allowed to carry firearms. They use their weapons only in exceptional circumstances and their actions may need to be justified in a criminal court.