Malcolm Rifkind: It is now clear that the Prime Minister took us to war on a false prospectus

While the guns are blazing and troops are fighting in a war, it is a necessary and honourable principle that opposition parties give unequivocal support to those who are risking their lives on behalf of their country. But once the guns fall silent the Opposition has an equally strong duty to hold the Government to account, expose its failures and inconsistencies, and demand the fullest investigation of its errors. That is why Michael Howard and the Shadow Cabinet have been obliged to speak on behalf of the public in the growing demand for frankness from Tony Blair on the way in which he took this country to war.

Of course most, though not all, Conservatives supported the war. Like the public, they read the Government's intelligence dossier and assumed that Blair knew what he was talking about when he described Saddam Hussein as "a serious and current threat". But it is now clear that he took Britain into war on a false prospectus and the Iraq war will, rightly, haunt Blair for the rest of his premiership.

Over the next few months the focus of criticism for the Opposition should be in three areas. The first must be the gross misuse of the intelligence agencies to provide a character reference for the Prime Minister. There has been much comment on the content of the "dodgy dossier" but this is secondary to the decision to use the agencies and the Joint Intelligence Committee to try to enhance the credibility of the Government's case for war.

In his introduction to the September 2002 dossier, the Prime Minister admitted that it was "unprecedented" for the Government to publish that kind of document. I trust it will never happen again. I was in receipt of top-secret documents for five years, both as Minister of Defence and as Foreign Secretary. Neither I nor any previous Labour or Tory minister would have dreamt of publishing material in the name of the Joint Intelligence Committee.

That would have been to politicise the JIC on an issue that divided the nation. It would have been like asking the Queen to call for war against Iraq.

The second issue must be the 45-minute claim on which Blair only recently made the extraordinary admission that he had been unaware that this applied solely to battlefield weapons with a range of a few hundred yards. This is not good enough. The claim was not buried away in the dossier. In his own introduction, in a paragraph referring to Saddam's goal of regional domination, Blair refers to the claim that some Iraqi WMD could be launched in 45 minutes. If the Butler inquiry does not investigate how the Prime Minister was as ignorant on such a vital matter as was, with more justification, the rest of the nation, it will fail in its duty.

The third area where the Opposition and the country must demand action is on the need for a Franks-style inquiry into the whole decision to go to war. The case for such an inquiry is powerful. After the Falklands War such an inquiry was set up although the nation was almost totally united and the war had been an unqualified success. The Iraq war, in contrast, has bitterly divided the country and its aftermath continues to involve massive loss of life, mainly to Iraqis but also to coalition troops.

A decision to go to war is the most difficult but also the most serious decision that any prime minister will ever take. When it is done without UN endorsement, dividing the country and on an intelligence basis that is subsequently found to be unjustified, to deny a full inquiry becomes perverse.

It is also the best way of resolving the question of the Attorney General's advice on the legality of the war. Legal opinions, whether to private citizens or to governments, are usually private matters for good reasons. To publish them does create an uncomfortable precedent. But we live in uncomfortable times. If Blair concedes a full inquiry into the war, the Attorney General's advice could be part of the evidence submitted and could be kept confidential if need be. What the Government cannot do is refuse both publication and a full inquiry.

Tony Blair's henchmen have taken to calling Michael Howard opportunist because of his trenchant criticisms of both the Government's Iraq policy and the way that the committee under Lord Butler seems likely to address its responsibilities. Methinks it doth protest too much. Howard is doing just what a leader of the opposition should be doing. He is reflecting the public's concerns and holding the Government to account.

It is not just the Iraq war that will be seen as the fault line of this government. It will be the way it politicised the intelligence agencies; the 45-minute shambles; and its refusal hold a comprehensive inquiry into its actions. It's not exactly what we thought an ethical foreign policy would be all about.

Sir Malcolm Rifkind is Conservative prospective parliamentary candidate for Kensington & Chelsea and was Foreign Secretary in John Major's government