Al-Qaeda sympathisers have been trying to infiltrate the British security service MI5, the BBC has learned.
Whitehall officials confirmed what some had long suspected, says BBC security correspondent Frank Gardner.
But those with al-Qaeda sympathies had been weeded out during a six- to eight- month vetting process, officials added.
Meanwhile anti-terrorism police probing the 7 July London bombings say people who knew the attacks were being planned would face prosecution.
Officers at a Scotland Yard briefing said they continued to be very concerned by the intelligence picture, with 70 investigations continuing and some of the information received described as "very sinister".
The head of the Metropolitan Police's anti-terrorist branch, Peter Clarke, said 60 people were awaiting trial in the UK for terrorist-related offences.
But he warned: "This is unprecedented, and the flow of new cases shows no sign of abating - if anything it is accelerating."
MI5 believes, from polls, that around 400,000 people in the UK are "sympathetic to violent jihad around the world", said Frank Gardner.
Within that number 1,200 people have been identified as being activists the security service believe are engaged in acts of terrorism at home and abroad, he said.
The first anniversary of the bombings of London will be marked on Friday by a national two-minute silence.
The silence, at 1200 BST, is part of a day of events to commemorate the attacks and remember the 52 victims.
Thousands of applicants
Our correspondent says that as investigations continue, MI5 is expanding from its current level of 2,600 officers to an eventual 3,500, to cope with the terrorist threat.
It is also setting up eight new regional offices to build up a "richer picture" of what people are thinking and doing on the ground, he added.
The security service now publicly advertises for staff as part of a major expansion plan to combat terrorism, ending decades of secrecy.
Tens of thousands apply to join MI5 each year, with 400 making it through to final selection.
Applicants are first assessed by an outside agency, then those shortlisted invited to MI5's London headquarters, where their backgrounds and sympathies are intensively investigated for six to eight months.
Peter Clarke said the level of its counter-terrorist investigations had intensified during the past 12 months.
Officers were "reconstructing" events leading up to the 7 July attacks to identify anyone who might have known they were going to happen, he added.
"It is an immensely complicated piece of work.
"Did anyone encourage them? Did anyone help them with money, accommodation or expertise in bomb making?"
He said "many lines of enquiry" were being pursued in the UK and overseas.
Officers had taken 13,353 witness statements, there were 29,500 exhibits and more than 6,000 hours of CCTV footage - some of which still needed to be analysed, Mr Clarke added.
"A great deal of progress has been made, but much remains to be done."
At least two of the bombers had visited Pakistan and the suspicion was they met up with people connected to al-Qaeda and attended training camps, he said.
But Mr Clarke added: "The part of the country where these sorts of activities take place is very difficult for the Pakistani authorities and ourselves to get any detailed information about."
Ringleader Mohammad Sidique Khan may well have made his suicide video overseas, Mr Clarke said.
Nail-encased peroxide bombs, similar to hand grenades with nails attached, left by two of the bombers in their car at Luton station, could have been used to escape arrest if they had been stopped on the journey to mount their attack, he added.
Mr Clarke also said officers had disrupted at least three further attacks since 7 July, which would be the subject of criminal trials.
The inevitable secrecy surrounding counter-terrorism and legal restrictions on the reporting of impending trials meant the public remained ignorant of the scale of the terrorist threat, he added.
"That is a pity and it means we must guard against blame, recrimination, speculation or myths taking the place of solid public information."
He said despite an increase in resources, his department was running at or near capacity.
"There is a lot of intelligence to be investigated - some of it is very sinister. It is a very, very concerning intelligence picture."
He added that the profile of the terror suspect was changing, including "many" British people willing to mount attacks on fellow citizens.
"Another feature which is really very very concerning is the fact that so many of them are so young," he said.
We must guard against blame, recrimination, speculation or myths taking the place of solid public information
The defence of the capital often starts many thousands of miles away
Peter Clarke, Metropolitan Police http://news.bbc.co.uk/go/pr/fr/-/2/hi/uk_news/5142908.stm