British Home Secretary Amber Rudd said that the government will suggest new security treaty to keep the level of counterterrorism cooperation with the European Union (EU) after Britain leaves the 28-member bloc.

Rudd, who did not give further information on the proposed treaty, made the statement on Saturday, a day after an explosion took place Friday morning on a packed rush-hour carriage at Parsons Green subway station in west London, Xinhua news agency reported.

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An 18-year-old man was arrested earlier Saturday in the departure area of the port of Dover in connection with the attack on the London tube bombing, which left at least 29 people injured.

The arrest is “very significant” and the terror threat level remains “critical”, Metropolitan Police Deputy Assistant Commissioner Neil Basu said.

British authorities raised the country’s terror threat level from “severe” to “critical”, the highest possible, meaning they think another attack may be imminent

The Islamic State claimed responsibility for the fifth terrorist attack in Britain this year.

The Friday blast was caused by an improvised explosive device, which failed to properly detonate, police said. London transport authorities said Saturday that they have re-opened Parsons Green subway station.

Previous attacks in Britain this year at Westminster Bridge, London Bridge and Finsbury Park as well as a blast at an Ariana Grande concert in Manchester killed dozens of people and injured more than 150 others.