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'.
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