Is the Attorney-General fit for purpose? Lord Goldsmith’s advice on Iraq has already earned him plenty of detractors. But now MPs from across the parties and even two ministers — Lord Falconer of Thoroton and Harriet Harman, QC — have joined them. If they haven’t come to bury their colleague, they want to take the knife to his office.

Today MPs will sharpen those knives more purposefully. The Constitutional Affairs Committee begins an inquiry into the Attorney's office and its concerns are pressing: decisions are imminent in the “cash for peerages” affair.

The furore is the latest to expose inherent tensions in the Attorney’s role. It falls to him, by statute, to consent to any prosecution over cash for peerages. And it is a role, he says, he cannot relinquish. Equally, he says, he will make up his mind with no eye to political fallout. Many attest to his integrity. But is such a stance possible — or credible?

The question goes to the heart of what the Attorney-General does. He wears several hats: he and the Solicitor-General are the Government’s chief legal advisers. They superintend the CPS and other prosecuting authorities and act quasi-judicially, bringing legal proceedings in the public interest.

The 500-year-old role is unique. Successive Attorneys have prided themselves on making decisions with political hats off. They remember with a shudder Sir Patrick Hastings in 1924: a belief that he dropped a prosecution on Cabinet instructions brought down the Government.

Pressures can be subtle and insidious. How well Lord Goldsmith withstands them is one thing. But how free, in turn, are the prosecuting authorities from political influence? If there was a dispute between the Attorney and the Director of Public Prosecutions, Lord Goldsmith has made clear his view would prevail.

Not surprisingly, there is little public confidence that a decision on cash for peerages will be impartial, soon after the Serious Fraud Office decision to drop its investigation into arms sales to Saudi Arabia. Maybe Lord Goldsmith can create a Chinese wall in his mind. But he must also be seen to do so.

There are calls for his role to be axed or made constitutionally independent. Conversely, some say he should remain as legal adviser within government and argue the clear need for a minister accountable for the prosecuting authorities.

But if Lord Goldsmith’s office is not to be utterly discredited, two reforms are urgently needed: he must stand aside permanently from any prosecution of a politician, adviser or civil servant; and legal advice to ministers must be disclosed on issues requiring a parliamentary decision.

The Government zealously dismantled the Lord Chancellor’s office because of his three hats. The Attorney-General’s position is even less sustainable.

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