With every disastrous revelation — on foreign prison records, absconding terrorism suspects or migration mix-ups — it is clear that John Reid’s damning assessment on taking over the Home Office is largely true. This does not mean that all those working there are unfit for purpose but it does mean this oldest Cabinet department is no longer suited to take on the responsibilities that it has accumulated since its foundation in 1782.

Mr Reid has now concluded that simply improving failing systems and agencies does not go far enough. What he now envisages is something altogether more radical: splitting his department into two, and creating instead a department of national security and a department of justice.

The first would assume responsibility for the police, passports, ID cards, border security, counter-terrorism and the intelligence and security services. In many ways it would be similar to the US Department of Homeland Security. The second would group together crime, prisons, the Probation Service and criminal justice policy, subsuming the present Department for Constitutional Affairs (DCA) and looking more like the justice ministry found in most continental governments.

This would be the biggest shake-up of any government department for years. It raises several obvious objections. The first is that changing the framework must not become a substitute for changing the substance. The reform of the Probation Service, an end to bureaucratic inefficiency, more targeted counter-terrorism and a clearout of the woefully incompetent immigration service are all urgent, and must all be followed through, however many further shortcomings they reveal. The second is that, without close co-ordination, the two new departments could be even more confused than the present sprawling hotchpotch. The police, for example, are key to both national security and the enforcement of justice.

There are, nevertheless, potential advantages in the change. National security has become a far greater challenge to all governments. It is not simply the threat of terrorist bombs: mass illegal migration and fraudulent asylum applications, unless controlled, can create social tension. Transnational organised crime is often beyond the control of national police forces. The issues of security surveillance, intelligence and government misuse of data provoke impassioned debate on liberty and security. Equally, prison overcrowding, lapses in record-keeping and poor monitoring of criminals can sabotage the administration of justice. The DCA, the clumsily named successor to the Lord Chancellor’s Department, should be transformed into a focused ministry that makes the delivery of justice swifter, more effective and, if possible, cheaper.

Two things should determine any plans for change: cost and logic. In this case, the first need not be high. The Treasury may quibble, but if Mr Reid wins the backing of Gordon Brown as Prime Minister the money will surely arrive.

The logic is more compelling. The Home Office is currently overstretched. It needs to get right, very quickly, its responses to national security challenges. And, separately, the justice system must be able to cope with those responses: building more prisons is only the first long overdue step. Yet before the divide-and-conquer strategy is embarked upon, Mr Reid must be absolutely clear about the deliniation of responsibilities and the creation of a culture of co- operation between the two departments. Without this, two Home Offices will not be better than one.