By Danny Shaw BBC News home affairs correspondent

Each constabulary has come to have its own identity

The 43-force structure has been in place for more than 30 years.

Each constabulary has its own identity; each chief constable has his or her own views on the best way to police; and each police authority has an eye on local politics and community feeling.

That is why successive home secretaries - Michael Howard, Jack Straw, David Blunkett - effectively ducked the issue, even though advisers might have tried to persuade them that re-organisation made sense.

Detailed study

But when Charles Clarke took over, in December 2004, it was the one major element of police reform which stood out above all others.

The Inspectorate of Constabulary had already touched on it in a report about the gap in services for what is known as "level 2" crime.

This category covers organised crime, murders and terrorism, as well as civil contingencies, roads policing and critical incidents.

In April 2005 the Home Office commissioned the Inspectorate to carry out a second, more detailed, study to see whether police forces could organise their protective services better to tackle these "level 2" crimes.

The report was published in September - just two months after the London bombings.

Mr Clarke reckoned that by asking police forces for their preferred options first it would be easier to achieve consensus Most forces 'against plan'

With the possibility of further terrorist attacks Mr Clarke was keen to waste no time.

He immediately adopted the report's central conclusion - that bigger equals better - and set police authorities a deadline, 23 December, to come up with detailed merger plans.

Any delay and the impetus for reform would be lost.

Mr Clarke reckoned that by asking police forces for their preferred options first it would be easier to achieve consensus.

He was also aware that politically sensitive issues which arouse strong local feelings are better dealt with well before a general election.

However, he cannot have foreseen the storm of protest that's followed from some chief constables, authority chairman and opposition MPs.

Compromise?

Lack of consultation time, worries about how the new arrangements will be financed, and serious doubts by some that mergers are necessary are their chief concerns.

Mr Clarke was keen to waste no time after the London bombings

The main problem for Mr Clarke is that only 13 forces favour merger, on the Association of Police Authorities' figures.

And of the proposed mergers none has the complete agreement of all the police forces affected.

Of course, some of the 13 forces who prefer to remain independent may be willing to compromise, especially if the Home Office finances the restructuring.

And some forces who have yet to declare their hand may come round to the merger idea later.