There are moments when a statement of the obvious cuts through the fog of self-interest and evasion that clings to much of politics, and clears the way for a genuine fresh start. The High Court's stunning condemnation of the decision to abandon an investigation into alleged bribery by BAE Systems is such a moment. “No one,” Lord Justice Moses and Mr Justice Sullivan declared, “whether in this country or outside, is entitled to interfere with the course of our justice.”

It should never have fallen to their lordships to point this out. They did so after concluding that the Saudi Government issued a “blatant threat” to stop sharing intelligence with the UK if the Serious Fraud Office did not scrap its long-running inquiry into alleged corruption linked to the £43 billion Al-Yamamah arms deal. Instead of defying the threat, British law enforcement, the British Government and at least one British ambassador surrendered to it. Fifteen months on, the damage to the rule of law and the reputations of the SFO and BAE is substantial and humiliating, but at least it can now start to be repaired. The same may not be true of the damage to Tony Blair's legacy.

Mr Blair has explicitly cited the Saudi threat as the reason for his defence of the SFO's decision. That threat may have been real, even if its significance has since been exaggerated. At the time, a year after the 7/7 bombings, it certainly gave Mr Blair better political cover than the need to safeguard further BAE contracts for 72 Typhoon figher jets, which were also at stake. But he failed entirely to acknowledge the risk that dropping the investigation posed to the international standing of British justice - a risk that the OECD and the United States have since underlined by launching their own inquiries.

Lord Goldsmith, the Attorney-General then, emerges little better from the affair. As the Government's chief legal adviser he should have pointed out that democratic governments seek to influence the judicial process at their peril. Instead, by meekly endorsing the Prime Minister, Mr Goldsmith came to embody a shameful blurring of the demands of justice and convenience.

Robert Wardle, the SFO director, took full responsibility for his decision and personally sought diplomatic advice before making it. The British Ambassador to Riyadh is reported to have warned him that British lives were potentially at stake should intelligence-sharing with Saudi Arabia cease. Hence Mr Wardle's conclusion that he was “powerless” in the face of the Saudi stance. Hence, also, the High Court's blistering riposte: “So bleak a picture of the impotence of the law invites at least dismay, if not outrage.”

This ruling demands urgent action of its targets. The SFO should reopen its investigation - or make public the concrete reasons for abandoning it. Gordon Brown should defend justice where his predecessor jeopardised it, by refusing to interfere and explaining, if necessary, that it would not be in his power to do so anyway. The Attorney-General needs to clarify how two sometimes incompatible roles will be combined in the future. And BAE, which has throughout the Al-Yamamah saga insisted it is blameless, should pledge publicly to reform a corporate culture that at the very least has depended on too cosy relationships with Government.

Whatever the implications of this ruling for Anglo-Saudi relations, the long-term health of British justice is more important. It is the bedrock of British civilisation - and prosperity.