John Reid, the Home Secretary, is facing fresh Cabinet opposition to his plans to split the Home Office and create a ministry of justice. The Attorney-General, the Government?s chief law officer, is believed to be fighting a rear-guard action to prevent the break-up, which he fears would hamper the fight against crime.

The original overhaul envisaged prisons and the probation service, now under the Home Secretary, moving to the Department for Constitutional Affairs to be with courts and legal aid in a new continental-style ministry of justice.

But The Times has learnt that Lord Goldsmith has concerns that placing the police in a new ministry for the interior will damage the close working relationship between police and prosecutors and make the criminal justice system less effective.

He also believes that there is a danger that courts will face increased pressure to tailor sentences according to prison capacity if they are in a department with prisons and the probation service.

One Whitehall official said: ?You are essentially putting the supply and the demand part of the equation in the same box. So if prisons are full, there could be a danger of pressure on the courts to stop jailing offenders or [to] jail them for shorter periods of time.?

The high-level opposition is a second blow to John Reid?s efforts to create both a ministry of justice and a new terrorism department with himself as ?security supremo? in charge of police, security services and terrorism.

He announced his plans only three weeks ago and has already lost the battle to put the police and security services together into a new ?antiterrorism? department.

Margaret Beckett, the Foreign Secretary, is reported to have threatened to resign if the Foreign Office did not keep control of the security services.

Lord Falconer of Thoroton, the Lord Chancellor, favours the idea of creating a ministry of justice and his supporters maintain that he could continue as head of such a ministry from his position in the Lords.

However there would be widespread opposition to the secretary of state in charge of prisons being in the House of Lords.

An alternative would be to create a Commons post for the new minister of justice and split off other duties of the Lord Chancellor, such as devolution and freedom of information, to another minister.

Baroness Scotland, a Home Office Minister who was cited last week as one of the most influential black women in Britain, has emerged as the hot tip to take over the slimmed-down role as Lord Chancellor. She would then be the first woman, and first member of the ethnic minorities, to hold the office.