Mike Granatt, former head of civil contingencies in the Cabinet Office and partner in Luther Pendragon Ltd, looks at the systems already in place in the event of a national emergency.
The emergency services would alert each other in a crisis
The events of 9/11 were a wake-up call in many countries including the UK.
Since then, there has been an effort - unprecedented since World War II - to improve the UK's resilience, increasing its ability to handle mass terrorism and other disasters.
But there is a lot of catching up to do.
True resilience involves capabilities for handling a wide range of contingencies, like emergency equipment and training; excellent intelligence and emergency services; well-prepared local authorities, health services, business and schools; and a well-informed public.
Rapid assessment of new intelligence is vital to protect against a wide range of possible terrorist threats.
The recently established Joint Terrorism Analysis Centre (JTAC) is run by MI5 and jointly staffed by experts from the police, Customs and Excise and other organisations.
It has a key role in delivering rapid co-ordinated advice on terrorist threats and is a real improvement on old single service arrangements. The model was quickly employed by other countries including the US.
Wartime sirens were dismantled years ago, so new ways are needed to deliver public warnings.
An extensive programme of multi-service, counter-terrorist exercises has been underway for more than 20 years
The major broadcasters, government, and emergency services and planners have new protocols to deliver warnings and information quickly alongside normal news programmes.
The BBC led the way with its Connecting in a Crisis initiative, which ensures that its local and regional stations can work closely with local emergency planners.
The new civil contingencies legislation will put stronger emergency planning duties on local authorities and responders.
And the government's new regional resilience teams will help ensure that resources can be moved in from unaffected areas.
Emergency exercises are a major contributor to resilience. An extensive programme of multi-service, counter-terrorist exercises has been underway for more than 20 years.
At least one of the many exercises held each year tests every level of response, including ministers, and can involve hundreds of people.
Levels of assistance
When a terrorist attack occurs, there is an immediate response from the local police and from the other emergency services. Not only do they send help to the scene, they alert other organisations who may be needed.
Drills are designed to test the response to a terrorist attack by all the emergency services
This will include the anti-terrorist branch, Specialist Operations 13 (SO13), government officials, the security and intelligence agencies, and the armed services.
A cordon will be set up around the scene to ensure that first and foremost, nothing slows the work to find and help survivors. It also helps keep the public from danger, and protects evidence.
A joint police, fire and ambulance control centre will usually be set up nearby. And in the case of a major and continuing incident, other centres swing into action.
At or near police headquarters, a major control room and co-ordination centre will be activated.
All necessary organisations will be brought together in a strategy group called Gold, chaired by the chief police officer.
The government will set up a similar group at ministerial level. Its role is to make sure that the chief police officer and Gold group have every resource they need. It is called COBR, the handily dramatic acronym for the Cabinet Office Briefing Rooms.
There, ministers and officials will keep in close touch with the Gold commander, ready to respond to requests for military aid or other support.
Finally and crucially, the public has to play its part in resilience, both in heeding official advice and in being ready to report anything suspicious.
Government booklets give guidance on prevention and planning for major accidents and terrorist attacks
The advice in the government's new booklet is deliberately simple, and it has been tried and tested.
If you are near to the scene, make sure you are safe and then report to the emergency services. You may need to be decontaminated quickly for your safety and that of others.
If you are further away but worried about an airborne hazard, go in, stay in and tune in. Go inside, shut the door and windows, and tune in to BBC or other local radio or TV for further advice.
But do not let any of this disrupt your normal life.
The threat of a terrorist attack somewhere may be high, but the risk that any of us face individually is much lower than the risk of crossing the road.
And we deal with that big risk well by taking precautions and following simple, effective advice.