Schoolchildren have been arrested by detectives investigating Russian and Iranian plots against Britain, a police chief has said, as he warned hostile state aggression was rising and youngsters were at risk.

Commander Dominic Murphy, head of the Metropolitan police’s counter-terrorism unit, said children in their “mid teens” had been investigated. It is understood they were suspected of being hired by criminals paid to carry out acts for Russia and Iran.

Russia, Iran and China are behind most of the hostile state action Britain faces, police said, which has increased fivefold since 2018, when Russian agents used the military grade nerve agent Novichok to try to assassinate a defector in Salisbury, Wiltshire.