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, The Telegraph reports.

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.

In a note the MoD had told personnel: "It remains possible that the perpetrator may attempt to place further devices."

British Transport Police confirmed that they had increased security arrangements because of the discovery and would be stepping up patrols.

The discovery of the suspect device and the subsequent arrest came 11 years after the 7/7 bombings when the London transport network was targeted in a terror attack that killed 52 passengers.

Details of the suspected plot emerged in the memo sent to serving military personnel, forwarding a warning from the security agency that monitors terror threat levels.

The memo said the Joint Terrorism Analysis Centre "has raised the threat level for transport in London to severe: an attack is highly likely".

It said: "This increase is in response to the discovery of a suspected viable improvised explosive device (IED) on a London Underground train yesterday.

"It is unknown who placed the device and what their motivation was. Therefore it remains possible that the perpetrator may attempt to place further devices. The threat level will continue to be reviewed as further information is received."

It is understood the threat level for transport outside London remains unchanged at moderate.