Britain is on the highest terror alert with the military set to bolster security as police hunt the Parsons Green bomb “suspects”.

Prime Minister Theresa May has raised the threat level to critical, meaning another attack is expected imminently.

A manhunt is under way after the improvised explosive device (IED) sent a fireball through a packed London Underground train carriage in south west London during the Friday morning rush-hour.

Assistant Commissioner Mark Rowley suggested there may have been more than one person involved stating that police were “chasing down suspects”.

British Transport Police at Euston Station, London (Tim Ireland/PA)British Transport Police at Euston Station, London (Tim Ireland/PA)

In a statement outside Scotland Yard, the UK’s most senior anti-terror police officer said: “Somebody has planted an improvised explosive device on the Tube – we have to be open-minded at this stage about him and about potential associates.”

Mr Rowley said detectives were only aware of one device and refused to be drawn on details of the suspects because of the “covert” nature of the operation. He said: “We are only aware of one device, so we now have the remnants of that device, it’s been examined by our experts, we’re chasing down suspects.”

The Islamic State (IS) terror group has claimed responsibility for the attack through its Amaq news agency, according to the US-based Site Intelligence.

It is the second time this year that the threat assessment – made by the Joint Terrorism Analysis Centre – has been raised to the highest level.

It was placed at critical in May in the wake of the Manchester bombing, before being lowered back to severe four days later.

Before this year the most serious category had only been reached on two other occasions since it was first publicly disclosed in August 2006.

A forensic officer outside Parsons Green station in west London as a manhunt has been launched after a bomb was detonated in a terror attack on a packed London Underground train (Jonathan Brady/PA)A forensic officer outside Parsons Green station in west London as a manhunt has been launched after a bomb was detonated in a terror attack on a packed London Underground train (Jonathan Brady/PA)

Rita Katz, the director of Site, said IS claimed the bombing was the work of a “detachment” rather than simply a “soldier”, which she suggested implied it was a co-ordinated attack.

Although it released a formal communique for the attacks, which injured 29 people, it provided no details on who the attacker or attackers were, Site said.

The terror group has frequently declared that it was behind terrorist incidents in recent years but experts have warned their claims should be treated with caution.

A bucket on fire on a tube train at Parsons Green station in west London amid reports of an explosionA bucket on fire on a tube train at Parsons Green station in west London amid reports of an explosion (Sylvain Pennec/PA)

As Britain faced up to another terrorist incident, following four attacks already this year:

:: Images emerged on social media appearing to show wires protruding from a flaming bucket inside a plastic Lidl carrier bag on the floor of a carriage;

:: Reports suggested the device had a timer;

:: A total of 29 patients needed hospital treatment, with a number of those hurt suffering from burns;

:: Scotland Yard and the Prime Minister rebuked US President Donald Trump over his claim the Parsons Green Tube bomber was “in the sights” of Scotland Yard.