The charge of genocide levelled this week against Sudan's President Bashir marks an anxious but important step for international justice. It will test not so much the fledgling International Criminal Court (ICC) as the integrity of the UN Security Council. It was the Council which set the justice process in motion, to cover its failure to stop mass murder in Darfur. Now it must stand by that process and require Sudan to respect the rule of international law that has resulted from it.

It was back in March 2005 that the Security Council, by Resolution 1513, referred the situation in Darfur to the ICC. By that stage, it had claimed some 300,000 lives and extirpated over a million refugees. The US Secretary of State Colin Powell had called it genocide, although an investigation set up by Kofi Annan doubted that the Bashir government had the requisite genocidal intent. However, its mass murder nonetheless amounted to a crime against humanity and it presented the Security Council with a list of 51 suspects, headed by Bashir himself. So in referring Darfur to the ICC, Council members were well aware that charges against the President could be the result. Russia and China did not demur, and the Bush administration abandoned its implacable and irrational opposition to the ICC to allow the reference to be made. Sudan's claim that the court has no jurisdiction over the members of its government is a legal nonsense.

It has taken more than three years for the investigation unleashed by the Security Council to bear fruit.The prosecutor, Luis Moreno-Ocampo, has gathered much more evidence than was available to Kofi Annan's inquiry and he says that it demonstrates a deliberate policy of tribal destruction, devised and ordered by the President himself. Ocampo is a careful, respected lawyer with a fine record in prosecuting inhumane Argentinean generals and also in handling defence cases: the Sudanese diplomats who have been defaming him this week are a disgrace. But his charges are merely allegations at this stage: under the ICC statute, they have no validity until he persuades three judges from the court's pre-trial chamber that they are likely to stick.

In fact, Ocampo is the world's least powerful prosecutor. The ICC was designed as a court of last resort, at a time (mid-1998) when President Clinton was reeling from the attentions of his own special prosecutor, Ken Starr. As Ms Lewinsky's blue dress was being DNA-tested, the US delegation evinced great fear of any ICC "super-prosecutor". To placate them, the court statute was drafted so as to severely tie Ocampo's hands. Unlike his equivalent in England, he cannot put anyone on trial without proving to a pre-trial chamber of three judges at a "confirmation" hearing that there are substantial grounds for believing in guilt. He cannot obtain an arrest warrant unless he can further prove that this is strictly necessary – which it is not in the case of a serving head of state who is prepared to cooperate with the court.

President Bashir is not at this stage prepared to co-operate: he falsely claims that the court lacks jurisdiction. His diplomats vilify the prosecutor while his ministers make bloodthirsty threats of reprisals against NGOs and indeed against the suffering people of Darfur. If any of this blackmail is carried into effect, then the Security Council must act – by force – to protect potential victims. It would be irresponsible to have set this procedure in motion and yet fail to protect those who are threatened because of it.

For that reason, the threatened reprisals are unlikely. China is Sudan's sponsor and protector, and it could not escape international obloquy, just before the Olympics, if it failed to restrain this client state. And any further brutal attack on the people of Darfur would serve to prove the prosecution allegation. However, Bashir and his ministers can make one perfectly proper counter-attack: they can send lawyers to The Hague to contest the charges at the pre-trial hearing that must now be held before this prosecution can proceed.

The confirmation hearing is held in public, and ICC rules permit absent "persons" (they do not become "accused" until the charge is confirmed) to be represented by counsel in the interests of justice. Sudan purports to be a responsible member of the UN and its ministers flatly deny charges brought against them in the UN's court as the consequence of a Security Council resolution. Their duty, then, is to make their denial in the proper and lawful way – not by vilifying the prosecutor and the court, but by taking the opportunity provided by the rules to contest the prosecution case before three independent judges, one from Africa (Ghana). There would be no question of arresting them unless or until the charges were confirmed.

International law directs Sudan to take this course and compliance with the Genocide Convention places a duty on states to ensure that it does. A few NGOS and many diplomats fear that taking the ICC route might derail a fragile peace process. But it would mean Bashir and his ministers remaining in place for the time being, and if the genocide charge is eventually confirmed it might be wondered how any peace that hinges upon their conduct could ever be assured. History teaches that peace, without justice for the perpetrators of genocide, is likely to be very short-lived.

Geoffrey Robertson QC is the author of 'Crimes Against Humanity: The Struggle For Global Justice'