The British police have arrested a man in the southern port of Dover in connection to a London commuter train bombing that injured 30 people a day earlier.
"We have made a significant arrest in our investigation this morning," Senior National Co-ordinator for Counter Terrorism Policing, Neil Basu said on Saturday.
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical," he added.
The arrest was made in the port area of Dover, where passenger ferries sail to France.
Prime Minister Theresa May put Britain on the highest security level of "critical" late on Friday after the blast on the London tube train at the Parsons Green underground station.
At "critical" level soldiers and armed police are deployed to secure strategic sites and hunt down the perpetrators of an imminent attack.
"For this period, military personnel will replace police officers on guard duties at certain protected sites," May said in a televised statement.
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Fifth terrorist attack in UK
The blast of a home-made bomb, which apparently failed to detonate fully, during the Friday morning rush hour on the west London tube was the fifth major terrorism attack in Britain this year.
The blast from the improvised explosive device caused burns for some passengers while others were injured in the blast's stampede. Health officials said none of the injured were in serious condition.
The terrorist attack was claimed by Daesh Takfiri organization, which is in war against the Iraqi and Syrian nations in the Middle East, but has also conducted Takfiri operations in other locations across the globe.
Daesh claimed other attacks in Britain this year, including two in London and one in Manchester.
Police have questioned these claims, saying that while the perpetrators of these attacks may have been inspired by the Takfiri ideology, there is scant evidence that the organization directly conducted the attacks.