Jim Gallagher: Eight goes into three very nicely, minister

It's not my job to advise you on the politics, but you heard what the First Minister had to say about avoiding unnecessary rows while his mind was on higher things. There is certainly a row brewing here.

The local authorities - including some of your own people - are up in arms about losing another responsibility to central government. The police themselves are almost all opposed. The main opposition parties had supported the idea of a single police force, but they have seen the results of the consultation too.

Of course, some of the council opposition is self-serving: was there ever a level of government that wanted to lose, rather than gain, more functions ? But there are real problems with the governance of a single force, and some serious risks in having one. I set those out for you a couple of weeks ago.

Would it work effectively enough with other local bodies in reducing crime and responding to neighbourhood concerns? How well would it respond to local preferences, rather than national targets? And what about the risks of politicisation and loss of operational independence (not under you, of course).

Now we have answers, we can offer to some of these criticisms, but they do give campaigners against a single force some real arguments. So can we avoid all the political trouble and still make progress here? I think so. Here is how.

First, remember what your promise was - to reduce the number of police forces, not to create a single force. A single force was Labour's policy, not yours. We can meet all the legitimate criticisms and still get the benefits of change if we go for three forces rather than one.

Why three? First of all, having eight forces, with one covering half of the population, is an accident of history. It goes back to the Wheatley report of 1968 and boundaries drawn in 1972. The world has moved on since then, and all other council services use the 32 boundaries created in 1995. Even if that were a more sensible number like 15, those areas would still be too small to support an efficient police service.

Historically, the trend has been to larger forces, as the complexity of policing has increased and more specialisation has been needed. So some of our forces now look too small. But Strathclyde is very big. The Metropolitan Police is a good deal bigger, but I would hesitate to recommend it to you as a model for Scottish policing just at the moment. Three forces would give a good balance.Boundaries are actually the least important issue, but we may as well start there: it's simple enough - West, East and North, in order of size.

There are really only two choices to be made. Is Tayside in the North or the East, and does Dumfries and Galloway get eaten by Strathclyde to form the West?

Tayside should go East: with the three others, that makes a good-sized force comparable to Strathclyde now, easily capable of looking after its own needs. It would look a bit strange to add Dumfries and Galloway to that, so it should become a division of the new West of Scotland force. Combining Northern and Grampian would create a smaller force in terms of officers, but with a very large, mostly rural, land area to cover. It could work very well. The important question is how these forces should be led and governed. Here there are three things we need to do to make things better.

First, we need to make the new joint boards work better, so that they are a real challenge to chief constables. Their performance is patchy now, but when the best of them work well they can be highly effective. We need a programme of support and training to get them all up to that level.

Second, we should take forward the plans you already have, to make the police service at divisional level relate formally to the individual local council. This already happens in most places, and works reasonably. It needs to become an established routine everywhere.

Third, we need to look again at your powers. The responsibilities you have were developed long before devolution, when your predecessors spent much of their time in Westminster and there was no Scottish Parliament to account to. You need powers that are appropriate today, and to answer to Holyrood for how you exercise them - and not for every operational mistake in the police. Getting these right will give you many of the benefits of moving to a single force without all the grief, and many of the risks.

You should have the explicit power to set national priorities for the police service, and the chief constables should be obliged to listen to what you say - and not just do so out of the goodness of their hearts. You also need powers to require them to provide services jointly, so that we can improve performance and cut costs. Chief constables have dragged their feet on shared services for too long, and they need to set to and save some money there.

The present Scottish Police Services Agency should be left alone: much of what it does ain't broke, so let's not fix it, and it is beginning to get to grips with the management of forensic science services. We should, however, change the governance of the Scottish Crime and Drug Enforcement Agency, which sits uncomfortably within a quango.Instead, you should use your new powers to require the three chief constables to maintain this agency as a joint enterprise between them.

Will all this work ? It stands a much better chance than moving to a single force. If we present it correctly, councils will get behind it, and so will chief constables.

More important, it will be an incremental change rather than a big bang, and the risk of impacting on performance is much less. The four forces in what will become the East of Scotland Police are already working together more, so we can build on that. The work we have already done shows that almost all of the savings we need to make do not depend on having a single force, but on making change happen. We have struggled to do that within the present structure, but this will open up opportunities - backed up by the threat from stronger ministerial powers. It should also be possible to get started quicker, with the three groups of forces rationalising some activities now.

What about police officer numbers ? These changes will help save money, but most of the police budget still goes on paying officers. You can only deliver on the First Minister's promise to maintain officer numbers by getting enough resources out of John Swinney. I'll be right behind you.

The most difficult challenge will be presenting this. Although the manifesto only promised to reduce the number of forces, you have gone courageously out on a limb and been identified with a single force. Let's make sure your colleagues don't saw through that branch with you sitting on it.

When the Cabinet meets to discuss this, you should present a three-force structure as your plan, which gets us almost all the benefits that moving straight to one force would have done, with less operational and political risk. What you have achieved by floating the idea of one force is acceptance of radical reform by a service which last year would have resisted any change whatsoever to the death. Who knows where it might lead in the longer run. Another triumph.

• Prof Jim Gallagher is a fellow of Nuffield College, Oxford, and was head of the Scottish Justice Department from 2001 to 2005.