THERE has been a heightened awareness of the threat of a terrorist strike on the UK - and London in particular - since the attacks on the US on September 11, 2001.
In 2002, Prime Minister Tony Blair said the threat of attack from al-Qaeda was "real and serious".
In March 2004, then Metropolitan Police commissioner Sir John Stevens described an attack as "inevitable", and said police had foiled several attempted attacks on the capital.
Before yesterday's blasts, officials had stressed the public should be alert, not alarmed, and insisted the capital had been well-drilled to face security incidents.
They said the aim must be to manage risk, saying it is impossible to eliminate risk altogether.
However, the opposition Conservative party has previously called preparations against an attack "half-hearted", saying workplaces should be made to rehearse for chemical and nuclear attacks as well as fires.
* Security spending
Spending on security rose from £950m ($1.7bn) a year before 2001, according to Chancellor Gordon Brown, to £1.5bn in 2004-05 to reach £2.1bn by 2007-08.
So far, £56m has been spent on mass decontamination units for use in chemical, biological, radiological or nuclear (CBRN) attack. Tens of millions of pounds in additional spending have also gone to the fire and health services, and to police counter-terrorism units.
However, relatively few people have been convicted of terrorism-related attacks.
In September 2003, firefighters donned bright green decontamination suits and police played the role of members of the public as they rehearsed the response to a chemical attack in an underground tunnel close to Bank Tube station.
This drill is said to have picked up problems including masks interfering with radio communications.
In February that year, tanks and hundreds of police and troops were deployed to Heathrow airport after intelligence reports suggested militants might be plotting a missile attack on a passenger plane.
* Parliament security
More recently, a series of stunt attacks on parliament has prompted questions about the security arrangements there.
In May 2004, campaigners from the Fathers 4 Justice group invaded the House of Commons to pelt Tony Blair with flour bombs, and fox-hunt supporters stormed the building in September.
A month later, a leaked Commons report called for heightened security measures around parliament, including electric fencing and a floating barricade on the River Thames.
Earlier this year, a protest exclusion zone came into force around parliament, prompting commentators to ask whether the right balance had been struck between security and the preservation of the right to free speech and protest.
Measures already implemented include security walls around parliament and Scotland Yard, and more rigorous visitor search procedures.
London Resilience is an inter-agency team set up following 9/11 to co-ordinate sectors including the emergency services, utilities, health, transport, and business and review preparedness for an attack.
It found London's emergency response arrangements were "well prepared for the types of major incident that had been considered previously. However work was needed to address the scale and nature of new threats."
Following yesterday's apparently co-ordinated explosions across the capital, the Metropolitan police commissioner Sir Ian Blair said it was a situation "for which we have planned and prepared".
He said: "A very significant and sophisticated emergency operation is now swinging into effect."