A 13-year-old boy has been arrested in Britain for a terrorism offence as part of an ongoing investigation related to the alleged sharing of extreme Islamist material online, Scotland Yard said here on Wednesday.

The Metropolitan Police said its Counter-Terrorism Command arrested the teenager at a west London address on Tuesday on suspicion of dissemination of terrorist material contrary to Section 2 of the UK's Terrorism Act 2006.

"The investigation relates to the alleged sharing of extreme Islamist material online," the Met Police said.

If he goes on to be charged and convicted, the teenager will become the youngest person in the UK to be found guilty of a terror offence.

"While it is still very rare for such a young person to be arrested for a terrorism offence, in recent times we have seen a worrying increase in the number of teenagers being drawn into terrorism, said Commander Richard Smith, head of the Met's Counter Terrorism Command.

"This particular investigation remains ongoing, but more broadly, we work closely with a whole range of partners to try and protect and divert young, vulnerable people away from extremism and terrorism, he said.

The boy was detained under the Police and Criminal Evidence Act (PACE) and taken into custody before being bailed to appear in court in mid-June. A PACE warrant to search the west London address was obtained by officers and has now been carried out. The Met Police said its officers will work closely with partners from child safeguarding agencies as the investigation continues.

Commander Smith said the public have an important part to play, as he urged anyone who thinks a friend or relative is becoming radicalised or drawn into a path towards terrorism to contact the force as part of the ACT Early strategy or Action Counters Terrorism Early. There is also an app called iREPORTit for people to report such terror related concerns.