There has been a heightened awareness of the threat of a terror attack on the UK - and London in particular - since the assault on the US on 11 September 2001.
Spending on anti-terrorism and attack preparedness has multiplied
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 intercepted several attempted plots.
Before Thursday'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.
Spending on security rose from an annual £950m ($1.7bn) 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.
There have been some highly visible examples of increasing readiness.
Security at parliament has been increased and more plans are afoot
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.
A report on the drill said improvements had been made but that more work was needed, particularly in preparing specific alternative plans for rescuing people underground. It also found problems in communications, including masks which interfered with radios.
Ministers said the exercise had been very valuable.
In February 2003, 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.
An inter-agency team, London Resilience, was set up following 9/11 to review preparedness for an attack and co-ordinate sectors including the emergency services, utilities, health, transport, and business.
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 Thursday'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".
"A very significant and sophisticated emergency operation is now swinging into effect," he told the BBC.
Experience of history
Three decades of attacks by the IRA in England - there have never been attacks in Wales or Scotland - mean authorities and civilians are well-versed in maintaining a certain level of vigilance and preparedness against attack.
But Islamic extremists, considered by many to pose the greatest threat today, may strike different targets and use different techniques.
A two-year study by eight leading academics in February found that there were holes in security in the UK's transport sector, emergency services were underfunded, the private sector was largely underprepared and that regions outside London were falling behind in preparing for an attack.
The authors - led by Paul Wilkinson, of the Centre for the Study of Terrorism and Political Violence at St Andrew's University - withheld sensitive findings on weaknesses in the "national critical infrastructure". This term is used to cover strategic energy supplies, the financial sector, and government administration.
A series of stunts 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.
Measures already implemented include security walls around parliament and Scotland Yard, and more rigorous visitor search procedures.
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.
Similarly, some critics accuse the government of using the threat to push through unpopular measures such as control orders and ID cards.
The study by the eight academics warned against allowing counter-terrorist policies to become a "party political football".
But the report confirmed the threat from al-Qaeda is "genuine and serious", and notes that some plots have been foiled.
The official terrorism threat level was lowered slightly from "severe general" to "substantial" at the end of May but at the time security sources stressed there was still a real and serious threat to UK interests.
It is understood the threat level has now been raised again.