Theresa May said the threat of terrorism is "severe" but it can be defeated by working together.
The UK's terror threat level was today raised to critical, which means an attack is imminent, hours after an explosion on a London underground train injured at least 22 people.
British Prime Minister had earlier said the threat level will remain at severe, which means an attack was highly likely, but that was revised by this evening as security forces continue their hunt for a suspected "bucket bomber" who detonated an improvised explosive device on the Tube at Parsons Green in south-west London, injuring 22 people.
"The threat level remains at severe. That means that a terrorist attack is highly likely. But this will be kept under review as the investigation progresses," Theresa May said after chairing a meeting of the emergency COBRA committee at Downing Street.
"The public should go about their daily lives but remain vigilant. People who are travelling in London will see an increased armed police presence on the transport network and security will be increased," she said.
May said the threat of terrorism is "severe" but it can be defeated by working together.
"We do need to ensure that we are dealing with, not just the terrorist threat but with the extremism and the hate that can actually incite that terrorism.
"That is why we are looking very carefully at the powers that our police and security service have to make sure they have the powers they need. But it is also why we are working with the internet companies," the prime minister said.
Scotland Yard said its inquiries into the terror incident remains ongoing and called on the public to remain vigilant.Assistant Commissioner Mark Rowley, head of national counter-terrorism policing said: "There are many urgent inquiries ongoing with hundreds of detectives involved, looking at CCTV, carrying out forensic work and speaking to witnesses".