John Reid, the Home Secretary, is poised to be given the role of terrorism and security supremo after apparently winning his battle to split his department in two.

Despite angry opposition from other departments and within Whitehall, Tony Blair has backed Mr Reid’s plan for him to be given full control of security, terrorism and policing.

The secret intelligence service (MI6) is said to be deeply unhappy that in future it will report to the interventionist Mr Reid rather than as at present to the Foreign Secretary. But the Home Secretary believes that the shake-up is needed to improve Britain’s overall capacity to counter terrorism and has warned his colleagues that delay will give advantage to al-Qaeda and other Islamist extremists.

Other parts of Mr Reid’s Home Office, including prisons and probation and criminal justice policy, are to move to the Department for Constitutional Affairs, which will effectively become a Ministry for Justice.

Lord Falconer of Thoroton’s Constitutional Affairs Department has registered the domain name justice.gov.uk and there have already been meetings between officials from both departments over future arrangements and posts.

Lord Phillips of Worth Matravers, the Lord Chief Justice, has also talked to the Home Secretary about the plan, which will in effect create a continental European-style ministry of justice and a ministry of interior, headed by Mr Reid.

Discussions have taken place within the Home Office over which staff will move to Lord Falconer’s department and on future designs for departmental logos and notepaper. The move has been resisted strongly by senior Whitehall civil servants, who resented that Mr Reid’s proposals appeared in the press before being discussed by the Cabinet.

Senior directors in the Home Office knew nothing of the plan until they read about them in Sunday newspapers last month. It is understood that both Sir Gus O’Donnell, the Cabinet Secretary, and Sir David Normington, the Permanent Secretary at the Home Office for only 12 months, had reservations about the idea.

The announcement of the split in the 225-year-old Home Office is unlikely to come before next week because a mass of details remain to be settled. But the decision in principle appears to have been taken.

A key factor is Gordon Brown’s apparent refusal to take sides in the internal government battle. The Cabinet Office and the Foreign and Commonwealth Office are opposed to the shake-up, but would have needed the strong support of the incoming prime minister to defeat it.

Mr Reid is expected to retain the title of Home Secretary because the Prime Minister has shied away from the more grandiose “secretary of state for security”, or even the American-style chief of homeland security. The shake-up will bring to fruition a long-held idea of the Prime Minister, who has frequently toyed with creating a ministry for justice.

In a Cabinet reshuffle in 2003 David Blunkett, who was then Home Secretary, defeated a proposal for the title of the Department for Constitutional Affairs to include the word justice. Mr Blunkett was determined to keep control of criminal justice policy from a Department for Constitutional Affairs.

Last night senior Whitehall figures said that the overall changes would not be earth-shattering and that they are essentially moving the deckchairs.

Officials say that in ministerial discussions Mr Brown has appeared to be relaxed about them. They are unlikely to conflict with his plans for a big reform of the machinery of government if he takes over as prime minister.

He has plans for a national security strategy under a unified security budget. Under Mr Brown, the prime minister would have overall control of security strategy and make annual reports to Parliament.

In the past he has made plain that he sees the advantage of policing, security and terrorism strategy coming under the same roof of criminal justice.

How it splits

Home Office

Police

Serious organised crime

Counter-terrorism strategy

MI5

Immigration and nationality

Passports

Drugs

Antisocial behaviour

Department for Constitutional Affairs

Prisons

Probation

Criminal justice policy

Office for Criminal Justice Reform

Sentencing Victims

Times Recommends

• The fourth plinth art project: famous for 60 minutes
• Rubik has a new puzzle to irritate the nation
• Is it Radio 4 or Radio Bore?