John Reid's decision to make a speedy and high-profile intervention in the sentencing of Craig Sweeney has clearly irritated the Attorney General.

In a statement issued on Monday night, Lord Goldsmith said he would not respond to political pressure when deciding whether to refer Sweeney's minimum sentence to the Court of Appeal as "unduly lenient".

Sources in the Attorney General's office insisted yesterday that the Home Secretary had an entirely proper role to play in maintaining confidence in the criminal justice system. The Home Secretary was fully entitled to signal his concern that Sweeney's tariff did not reflect the seriousness of the crime - even if this did look like an attack on an individual judge.

But what was regarded as unhelpful was that Mr Reid went on to say he would be asking the Attorney General to consider referring the case to the Court of Appeal. The Home Secretary made this public less than four hours after Sweeney had been sentenced.

Knowing that the law officers would receive similar requests from members of the public, Lord Goldsmith had already called for the case papers. But the Attorney General would not want people to think he was susceptible to pressure from a Cabinet colleague when acting in his role as guardian of the public interest.

This is a difficult trick to pull at the best of times. The more the Attorney General takes part in overtly political activities, the harder it is for him to persuade the public that he is acting disinterestedly when he exercises his quasi-judicial role as a law officer.

The same goes for his deputy, the Solicitor General. Both have dual roles: they must advise and represent the Government, of which they are members, while also taking legal decisions - on whether, for example, to bring prosecutions - regardless of whether they are in the Government's interest. The Attorney General must also try to uphold confidence in the judicial system - not an easy task when the judges believe that their hands have been tied by recent Home Office legislation.

Under the Criminal Justice Act 2003, judges are required to pass a life sentence on someone who commits a "serious" offence if there is a significant risk that the offender will commit further offences causing serious harm to the public. Judges are then required to specify the minimum period after which the offender may be considered for release on licence.

Before calculating this minimum period, judges have to reduce the sentence by one third if the offender has pleaded guilty at the first opportunity. The Government-sponsored Sentencing Guidelines Council is currently consulting over whether this should be changed.

In Sweeney's case, Judge Griffith Williams, QC, started with a sentence of 18 years, in line with draft guidelines published last week by the council and a ruling by the Court of Appeal a day later. Taking a third off for the guilty plea, Sweeney was left with a sentence of 12 years.

This is the sentence that Sweeney would have received until the 2003 Act came into force last year. His victim's family would still have regarded it as too lenient, but there might have been less of a public outcry.

However, Sweeney would have been released automatically after serving half this sentence. This is purely a matter of Government policy, which Mr Reid could abolish overnight if he had twice as much prison accommodation. Because he would have served only six years of a 12-year fixed sentence, Sweeney's tariff was reduced to six years. This was further reduced to five years and three months to take account of the time he had spent in prison on remand.

Lord Goldsmith's statement emphasises that he has not yet decided whether to ask for Sweeney's sentence to be reviewed.

"He will only refer a sentence to the Court of Appeal if he believes that it falls significantly below what any judge could reasonably have passed," his spokesman said. Lord Goldsmith will consider the views of Treasury Counsel - highly experienced prosecutors - and legal advice from within his own office before deciding whether to refer up Sweeney's case.

He has four weeks to do so and a decision is expected early next month.