A terror plot to target the London Underground has been foiled after police found what they suspect to be a viable bomb on a tube train, planted by a terrorist planning further attacks.

A 19-year-old man was tasered and arrested by armed counter-terrorism officers on Friday, a day after the suspect device was found on board a train near the O2 centre.

On Friday it emerged that security officials had contacted the Ministry of Defence after the find, to warn serving military personnel, and the threat level for transport in London had been raised to severe to reflect that an attack was highly likely.