MI5 warned that disclosing why it failed to investigate the ringleader of the July 7 attacks could give al Qaida plotters an “invaluable weapon”.
Bereaved families want to use the forthcoming inquests into the deaths of those killed to ask security service officials why they did not follow up Mohammad Sidique Khan, of Dewsbury, after he was witnessed meeting known terror suspects.
But MI5 told a hearing at the Royal Courts of Justice in London to decide the scope of the inquests that allowing this would aid those planning the next atrocity.
Neil Garnham QC, counsel for the Home Secretary and MI5, described the 2005 London bombings as “the deliberate action of evil and callous killers”.
But he went on: “The appalling truth is, however, that there are people out there who applauded and celebrated this appalling act.
“And there are others who would like to repeat it, to emulate the action of these murderers and see to it that there are other innocent people bereaved like the families here.
“Not all such people are stupid. Some of them are both evil and intelligent.”
In early 2004 Khan was watched, photographed and followed by counter-terrorism officers during an inquiry into a group of extremists planning a fertiliser bomb attack.
But MI5 concluded that diverting resources to place him under detailed investigation or surveillance was not justified.
Khan, 30, and three other suicide bombers – including Huddersfield’s Jermaine Lindsay – killed 52 innocent people when they detonated explosives on three Tube trains and a bus in central London on July 7 2005.