BBC defence correspondent Paul Adams explains the background behind the idea for the force.

What will the force do?

A variety of functions. According to the discussion document, The Role of the Reserves in Home Defence and Security, tasks could include reconnaissance, assisting with mass casualties, site search and clearance, transport and communications, the operation of water and feeding points, the control of movement of large numbers of the public and guarding key facilities.

Who will be in the force?

The reaction force will be made up of reservists from all three services, but mostly the Territorial Army (TA).

An average of 500 volunteers will make up each force in 11 regions of the UK. The nationwide total is expected to be around 6,000.

The volunteers will remain in their parent units until they are called upon to act.

Why regional units? What is the point of using reservists, instead of regular armed forces personnel?

It is impossible to predict where a terrorist attack might take place, so it makes sense to have reaction teams dotted about the country, each with detailed knowledge of the immediate area, able to respond quickly to local emergencies.

The strength of the reservists, it is argued, is precisely because they are locally based.

The government does not rule out a role for regular units in responding to national emergencies.

The latest document looks in particular at the role of reservists, but it is recognised that every member of the armed forces could play a part in the event of a terrorist attack.

Why does the government say such a force is necessary?

The government believes the police and emergency services are already well-placed and equipped to deal with most critical situations, but recognises that some situations, described in the document as "wholly exceptional events", could place an unbearable strain on the existing structures.

Hence the need, it is argued, for a back-up force that could be called when extra help is needed.

How has the idea been received by reservists and politicians?

Where does it go from here?

The government has given all interested parties - primarily reservists and their employers - until 13 September to respond to the latest proposals.

Some consultations have already taken place and it is clear that the government is not expecting to hear serious voices of dissent.

Officials say they expect the idea to start becoming a reality by the end of the year.

The political response has been mixed, but generally favourable.

Supporters of the TA have welcomed a new role for a force that has felt somewhat neglected in recent years.

However, shadow defence secretary Bernard Jenkin, said a much bigger, more expensive force would be needed to make a real difference.

A number of sceptics have dismissed the government's proposals as window dressing.

Why doesn't the government set up a department of homeland defence to co-ordinate initiatives like this?

"We already have a director of homeland defence," officials argue, "and he's called the home secretary."

A slightly flippant remark which nevertheless highlights a key point about the latest proposals - the Ministry of Defence is not setting out to challenge the Home Office.

"The lead responsibility for domestic security," the document says, "lies with the civil agencies and with the police in particular."

The government's feeling, justified or not, is that decades of experience of IRA terrorism has honed the skills and organisation of Britain's security and intelligence services.

New structures, it says, are not needed - just a little more manpower.