Today’s announcement that the Home Office is to be split in two, after 225 years as one of the great offices of State, brings to fruition a long held plan of the Prime Minister.

The major Whitehall shake-up, which effectively creates a European style ‘Ministry of the Interior’ alongside a Ministry of Justice, was until today part of new Labour’s “unfinished business”.

The move towards the split, although suddenly disclosed in Sunday newspapers earlier this year, had its gestation more than four years ago.

Then, David Blunkett was able to block the proposal and he even managed to halt the word justice being included in the title of the newly former Department of Constitutional Affairs.

However, the idea was revived again in the aftermath of last August’s alleged plot to blow up a transatlantic flight when a review by John Reid concluded that a step change was needed in the UK’s counter terrorism strategy.

Mr Reid concluded that one minister should have overall control of the nation’s counter terror strategy, and clearly saw himself as the man to do the job.

He proposed that the Home Office should shed its responsibility for probation and prisons and criminal justice policy. Instead, it would concentrate on policing, immigration and asylum, serious organised crime and drugs, while the new Ministry of Justice would deal with offenders after they had been caught by the police, prosecuted, jailed and, hopefully, rehabilitated.

From today, a new office of national security and counter-terrorism started work in the Home Office and, from May, a new Ministry of Justice will open its doors, not only dealing with courts but also criminal justice policy, prisons, probation and youth justice.

Despite the gathering of more powers in the hands of the Home Secretary, the UK's counter-terror effort remains fragmented, with the Cabinet Office retaining its role in relation to intelligence, and the Foreign Office retaining responsibility for MI6 and GCHQ.

Lord Falconer of Thoroton, QC, will be the first Secretary of State for Justice but is likely to have only a few weeks in the job as Whitehall has made clear that, in the long term, it would be untenable for such a figure to be sitting in the House of Lords.

And there are also fears, among the police and some backbenchers, that the Ministry of Justice may effectively become a Ministry for judges and lawyers - a 'conservative with a small c' department.

It is also clear that, although one of the criticisms of the Home Office was that there was a lack of communication between different divisions, the same problems are just as likely to emerge with two departments.

After all, the police have a clear interest in those who are in prison and probation as well as criminal justice policy.

There is going to have to be a close working relationship between the two departments, and harmony between ministerial colleagues, or else there will be classic Whitehall turf wars.

Times Recommends

• Margot Fonteyn: In her footsteps
• Forget beautiful people, let's have wonky Brits
• Fundamentalist brew: faith and power