Scene of London Underground attack
Scene of London Underground attackReuters

Britain on Sunday downgraded the nation's terrorism threat from its highest level, after police made a second arrest in connection with Friday’s bombing of a London Underground train, AFP reports.

"The Joint Terrorist Analysis Centre, which reviews the threat level that the UK is under, have decided to lower that level from critical to severe," interior minister Amber Rudd said in a televised statement.

A critical threat level means another attack is "expected imminently" while a severe threat indicates an assault is highly likely.

The announcement came hours after police said that a 21-year-old man, who has not been identified, was detained late Saturday in Hounslow, on the western rim of the capital.

An 18-year-old suspect was arrested in Dover on Saturday.

The bomb went off during Friday's morning rush hour in a packed carriage and although the device is thought to have malfunctioned, it still wounded 30 people.

It was Britain's fifth terror attack in six months.

Rudd said the police were trying to find out how the first man arrested was "radicalized".

The Islamic State (ISIS) group claimed responsibility for Friday's explosion, though Rudd voiced doubt over that claim.

"It is inevitable that so-called Islamic State or Daesh will try to claim responsibility but we have no evidence to suggest that yet," she told the BBC on Sunday.