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 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.