Never underestimate the determination of a quiet man — or the power of a recently retired senior law lord.

Lord Bingham of Cornhill this week stoked the flames of the legality of Britain’s invasion of Iraq to devastating effect.

His critique — contained in the Grotius Lecture celebrating 50 years of the British Institute of International and Comparative Law — a detailed scrutiny of the rule of law in the context of international affairs, would have been well-received by his audience.

It also precipitated a swift response from two senior Labour figures who held key positions at the time of the invasion: Jack Straw, now Justice Secretary and then Foreign Secretary, and Lord Goldsmith, QC, then Attorney-General, who supplied the legal advice on which the decision to invade was made.

Not only that, just hours before the lecture was due for delivery on Monday night, Lord Bingham issued the media with an amended embargoed version of the text.

As a courtesy, days earlier, he had supplied the Justice Secretary and former Attorney with his original. The latter, in particular, wasted no time protesting that he had not been properly represented. “I should make it plain,” Lord Bingham told his audience, that both men “challenge the conclusion I have expressed”.

That conclusion, in line with a host of other legal commentators, was that the legal advice given by Lord Goldsmith was fundamentally “flawed”. First, it had failed to acknowledge the lack of hard evidence to indicate Iraq’s compliance with UN Resolutions.

Secondly, it did not say that military action had to be authorised by the United Nations Security Council. It “passes belief”, Lord Bingham said, that a decision as to whether Iraq had or had not taken advantage of the final opportunity to comply with its obligations to disarm “was intended to be taken otherwise than collectively by the Security Council”.

He concluded: “If I am right that the invasion of Iraq by the US, the UK and some other states was unauthorised by the Security Council there was, of course, a serious violation of international law and of the rule of law.”

In his amended text he acknowledges Lord Goldsmith’s arguments: that the former Attorney believed his advice at the time to be correct (which Lord Bingham points out that he had not challenged), and that the former Attorney remains of that view.

Lord Goldsmith made three further points on the issue of legality. First, that the use of force in 2003 was authorised by the UN because of its original earlier authorisation — and that remained in force. The argument had been relied on before and supported by UK law officers and was also endorsed by the Secretary-General of the UN in 1993.

Secondly, the Security Council had set out conditions in which permission to use force would be revived; and that did not require the Council to decide that there had been a further material breach.

Thirdly, the UK was justified in concluding that the final opportunity (to comply over disarming) had not been taken. Lord Goldsmith had advised Tony Balir, the Prime Minister, that he had to be sure; and the Government judged that it had not done so. Jack Straw agreed with those points, Lord Bingham told his audience.

Both men also issued statements to the media to coincide with the lecture. Jack Straw said that Lord Goldsmith’s advice that military action was lawful and in accordance with the Security Council resolutions “was shared by many member states across the world”.

The central issue was whether a second resolution was needed and whether “an additional lock requiring a further decision on military action should be included in Resolution 1441”. In other words, did Resolution 1441 revive the authority to use force if there had been a material breach of the ceasefire? Or was a further resolution needed?

In the event, Straw added, the agreed text of 1441 was in line with the US-UK position that the resolution “did not require this further lock”. It was, added Lord Goldsmith, a “very deliberate omission” in its wording.

Lord Bingham — as if hearing an appeal in the House of Lords — had allowed further arguments to be put to him. And as if giving a judgment, he fairly and accurately summarised them. But he reached the same conclusion as in his original text.

Straw, who had the task of addressing the dinner that followed Lord Bingham’s lecture, was courteous in his response and included a generous tribute to the former senior law lord as the “finest jurist of his generation”. He made clear, though, that he rejected Lord Bingham’s conclusions which, he said, did not “I’m afraid, take proper account of the text of Security Council Resolution 1441 nor its negotiating history”.

In some ways it is extraordinary that the debate has been revived now. Lord Goldsmith seemed to suggest that it was history: “Lord Bingham is entitled to his own legal perspective five years after the event, but at the time and since then, many nations other than ours took part in the action and did so, believing that they were acting lawfully.”

And he, of course, is no longer in government. Yet only now is Lord Bingham, 75, able to speak out, freed from judicial shackles. It is also a testament to his authority that his views carry such weight; neither Jack Straw nor Lord Goldsmith felt able to ignore them.

Timing, though, is all. Lord Bingham’s comments refuel debate over the Iraq war just as the Government seeks to finalise a deal with Baghdad over British troops remaining in the south of the Iraq.

So the legality of the invasion is not just a moot point for lawyers: it remains highly sensitive for ministers and five years on, continues to haunt.

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