Armed police have flooded London's streets as the terror threat level was raised to critical amid fears the Parsons Green bomber could strike again, Theresa May announced tonight.

The introduction of Operation Temperer will see soldiers replacing police at key sites including nuclear power plants to free up extra armed officers for regular patrols.

Scotland Yard said it is making 'excellent' progress in hunting the suspected terrorist who set off a crude bucket bomb on a packed commuter train by Parsons Green tube station in west London at 8.20am.

Mrs May said in a statement from Number 10: 'The Joint Terrorism Analysis Centre has now decided to raise the national threat level from severe to critical - this means their assessment is that a further attack may be imminent.'

Minutes later Assistant Commissioner Mark Rowley suggested there might have been more than one person involved, stating that police were 'chasing down suspects'.

Police identified the suspected terrorist using CCTV footage but the investigation has been overshadowed by an extraordinary diplomatic row triggered by Donald Trump

The US President tweeted just hours after the rush hour blast that police had the attacker 'in their sights' and should have been 'more proactive' in catching 'the loser'.

Scotland Yard hit back and said Mr Trump's comments were 'pure speculation' while senior officers refused to name the suspect.

The President later rowed back on his controversial comments by posting another tweet saying, ‘our hearts and prayers go out to the people of London’.

The Islamic State group claimed responsibility for the attack tonight, saying its 'soldiers' had 'planted IEDs'.  It was the middle of rush hour when the crude bucket bomb - which had a timer - went off at 8.20am inside a tube train packed with commuters, including children and a pregnant woman.

The device was hidden in a builder's bucket and could have killed dozens but failed to properly detonate and sent a 'wall of fire' through the carriage at Parsons Green, injuring at least 29 people.

Terrified passengers were left covered in blood with scorched hands, legs, faces and hair – others suffered crush injuries during a stampede as they 'ran for their lives' over fears the 'train would blow up'.

London Ambulance took 19 patients to hospitals, while the others went in themselves. The four hospitals dealing with patients were Imperial, Chelsea and Westminster, Guy's and St Thomas' and St George's.

Officers are tonight hunting for the bomber across London amid claims he could be armed and might have planted other explosive devices.

An officer at the scene told MailOnline: 'We believe there is a second bomb - there is a man with knives on the loose.'

In a pre-recorded television statement released around 8.30pm, May said military personnel would replace police officers 'on guard duties at certain protected sites which are not accessible to the public'.