The government has been advised on the issue by Lord Goldsmith, the attorney general. His advice is to be disclosed today. All the prime minister has been prepared to say so far is that the UK will not take any action that does not have a "proper legal basis", as he made clear in his answers in parliament last week.

Time is now running out. In the very near future British troops are likely to be committed to battle. They, their families and the public have a right to know what the "proper legal basis" for their action is. Engaging in armed conflict in breach of international law is a precarious business. The idea that the prime minister would end up before the international criminal court for participating in a US-led attack is far-fetched. But military commanders on the ground will not thank the government if any action they take is later judged to have been in breach of international law.

The limits of the potential arguments available to the government are clear. According to the UN charter, there are only two possible situations in which one country can take military action against another. The first is in individual or collective self-defence - a right under customary international law which is expressly preserved by Article 51 of the UN charter. The second is where, under Article 42 of the charter, the security council decides that force is necessary "to maintain or restore international peace and security" where its decisions have not been complied with. In other words, where a UN resolution clearly authorises military action.

The question whether the Article 51 self-defence route justifies a pre-emptive attack has been keenly debated. Article 51 itself is silent on the matter. But even if it does justify a pre-emptive strike, which is surely the sounder position in a nuclear world, any threat to the UK or its allies would have to be imminent and any force used in response to that threat would have to be proportionate before Article 51 can be relied on. The mere fact that Iraq has a capacity to attack at some unspecified time in the future is not enough.

The problem for the government is one of credibility. No one believes that Iraq is about to attack the UK or its allies, and any self-defence claim by the government would sit very uncomfortably with the US position that military action is justified to destroy such weapons of mass destruction as Iraq may have, and to bring about a change of leadership.

The second route, which depends on Article 42 of the UN charter, appears more promising for the government. There are two strands to this argument. The first is that resolution 1441 itself authorises the use of force against Iraq. It warns Iraq that "it will face serious consequences" if it continues to violate obligations spelled out in that resolution. But, critically, the words "all necessary means" have not been used.

They are important words because they are the formula used by the UN to indicate that the use of force is authorised. They were the words used to justify military action against Iraq in 1991 and, subsequently, when the security council authorised intervention in Rwanda, Bosnia, Somalia and Haiti. The argument that all the security council members, including France and Russia, intended to authorise the use of force when they voted for resolution 1441 is hardly compelling, and arguments that resolution 1441 implicitly authorises the use of force run into the same difficulty.

The only real alternative for the government is to argue that Iraq's failure to comply with the ceasefire requirements of UN resolution 687, passed at the end of military action against Iraq in April 1991, justifies the renewed use of force. But that, too, is not without its difficulties. Like resolution 1441, resolution 687 does not itself authorise the use of force. The only security council resolution expressly authorising the use of force against Iraq was 678, which was passed at the start of the Gulf war in November 1990, and the only action it authorised was such force as was necessary to restore Kuwait's sovereignty.

It is true that the ceasefire resolution 687 requires Iraq to destroy all weapons of mass destruction, but under Article 42 it is for the security council and not the US or UK to decide how it is to be enforced. In 1993 the UN secretary general suggested that resolution 678 justified US and UK air attacks to enforce the no-fly zone in Iraq. But that is a very fragile basis for arguing that, 10 years later, it justifies an all-out attack without the need for a further UN resolution.

The government has a point when it grumbles about permanent members of the security council, such as France and Russia, threatening to veto any further UN resolution. But that does not justify the US or the UK acting outside the UN. It merely highlights the need for reform of the undemocratic security council structure which they put in place at the end of the second world war. Article 2 of the UN charter requires all states to refrain from the threat or use of force that is inconsistent with the purposes of the UN, which emphasises that peace is to be preserved if at all possible.

Against that background, it is no surprise that the government has been coy about its advice so far. But on the eve of war that is not good enough. If the attorney general's advice is that force can be used against Iraq without a further UN resolution, he must explain fully how the legal difficulties set out above are to be overcome. Simply to argue that the interpretation of resolution 1441 accepted by all the other security council members except the US and the UK should be abandoned in favour of military action won't convince anybody. Flawed advice does not make the unlawful use of force lawful.

· Keir Starmer QC is a barrister at Doughty Street Chambers specialising in internat- ional human rights law. He is a fellow of the Human Rights Centre at Essex University.

k.starmer@doughtystreet.co.uk