British media is reporting that 12 terrorism suspects arrested across England were planning co-ordinate attacks on Christmas shoppers and government targets in central London.
Police and MI5 officers raided four homes across the UK and arrested men allegedly linked to a banned extremist group.
The suspects are all aged between 17 and 28 and were under arrest in coordinated raids in Cardiff, Stoke-on-Trent and east London.
Police have not confirmed the facts of the alleged plot, but it was said to involve attacks on shops, banks and "iconic targets" in London.
British police and security services have investigated dozens of suspected plots and under arrest hundreds of suspects since the September 11, 2001 attacks on the United States.
Jonathan Evans, head of the MI5 security service, said in a rare speech last year that his major task was tackling Al Qaeda-inspired militants. He highlighted a growing threat from militants in Yemen and Somalia.
Four suicide bombers killed 52 peoples on three trains and a bus in London in July 2005.