London, Sep 18: British Home Secretary Amber Rudd said that Britain's terrorism threat level has been lowered from "critical" to "severe" after being raised to the highest possible in the wake of the Friday explosion at a subway station in west London. Rudd made the statement on Sunday after British police arrested two suspects in connection of the explosion in a packed rush-hour carriage on Friday at Parsons Green subway station, Xinhua news agency reported. The level of "critical", the highest of the five levels used to describe the threat, means a further terrorist attack may be imminent. The level of "severe" is the second highest level. The threat level system, introduced on August 1, 2000, is based on available intelligence, terrorist capability, terrorist intention and timescale. The two suspects -- one is 18 and the other is 21 -- were arrested by British police on Saturday. Thirty people were injured in the explosion, none of them seriously, in the wake of the subway blast, prompting the police to stage a massive hunt for those who are responsible for the fifth terrorist attack in the country over the past six months. Previous attacks in London 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. The Islamic State has claimed responsibility for the Friday subway explosion.