How much does the public have the right to know about the decision-making that led to Britain going to war in Iraq?

Given that the conflict caused the deaths of 179 ­British service personnel and an ­estimated 460,000 Iraqis; that Iraq has suffered from dangerous internal discord ever since; and that certain claims used to justify the case for Britain going to war have been exposed as false, the argument for full disclosure is simply overwhelming.

That is why Sir Menzies Campbell, long the Blair Government's fiercest critic over Iraq, is absolutely right to describe Whitehall's foot-dragging over whether to declassify key documents as intolerable. The whole truth about Britain's involvement should now be told in the public interest.

It is four years since the launch of the Chilcot Inquiry into Britain's involvement in the war. The five-member panel's investigation is broader than previous inquiries and is intended to be definitive. Yet it has been harried by delays. The inquiry is held up because the Cabinet Secretary Sir Jeremy Heywood has yet to reach agreement with the inquiry chairman Sir John Chilcot over the inquiry's requests to declassify more than 150 letters and conversations between former prime ministers Tony Blair and Gordon Brown and the then-US president George Bush.

Sir John says he finds the delay regrettable. In the gentlemanly language of senior officialdom, that counts as stiff criticism. The delay is not only holding up the publication of the report; it is also causing uncertainty about what evidence will be available to support criticisms of key individuals, with Mr Blair expected to be among them.

If someone is to be censured in a government report, then the public must understand why those judgments are being made. To criticise without disclosing the supporting evidence would create more questions than answers. It would also be unfair on those whose actions are under the spotlight. If they wish to speak up in their own defence, they will find it hard to do so if the documents in question have not been made public. While this uncertainty drags on, they are left in limbo. The inquiry could take the stance of withholding censure altogether in the absence of supporting evidence, but if it did that then what would be the point of having it? It is clear that there should be a presumption of openness.

Due consideration must of course be given to the risk that releasing sensitive documents could harm the national interest. However, as Sir Menzies has said before, the panel can surely be trusted to decide whether documents should be made public "with due regard for the distinction between the national interest and private embarrassment".

Behind Sir John Chilcot's most recent polite letter to the Prime Minister is a man frustrated. He cannot speak as plainly as one suspects he might like, but Sir Menzies Campbell can. That is why the LibDem grandee will be so sorely missed from frontline politics when he retires in 18 months' time.