It was one of the longest counter-terrorism raid in nearly two years as police arrested the 12 suspects on Monday, Dec 20. At least five were of Bangladeshi origin.
Lord Carlile, the government's independent watchdog for terror legislation, said on Tuesday, Dec 21 the alleged plot appeared significant and involved several British cities, but he did not identify the targets.
Police have up to 28 days to either charge the men or release them.
Possible targets that were scouted include the Houses of Parliament in London and shopping areas around the UK, a security official stated.
(With inputs from PTI)