The UN security council was expected to back a resolution authorising the prosecution of Sudanese war crimes suspects by the court in a case that could prove crucial to establishing the court's legitimacy.

Prosecutors said in January they would welcome the Darfur case if they were given jurisdiction by the UN. But that was thought unlikely given US opposition to the creation of the court and its involvement in Darfur, where hundreds of thousands have been slaughtered and even more displaced over the past 18 months.

A House of Commons report released on Wednesday estimated the total number of dead in the region at "some where around 300,000" and accused the international community of a "scandalously ineffective response" to the situation.

The sheer scale of the conflict in Darfur and the danger of sending investigators into a conflict zone will cause difficulties for the court, which has yet to try a case. It will also be expensive, placing a huge strain on the court's £46m budget. But a successful prosecution would help establish its authority.

Michael Wladimiroff, a lawyer who defended the first suspect at the UN tribunal for the former Yugoslavia in the mid-90s, called the apparent shift in US policy an unexpected change that could open the way for further cases at the court, which is based in The Hague.

"This means the court ... can now be used as an instrument by the security council," he said. "All of a sudden there will be a change from waiting for cases to expanding capacity and moving more quickly toward trials."

The ICC can intervene only when countries are "unwilling or unable" to dispense justice themselves. Ninety-eight countries ratified its founding treaty in July 2002 to prosecute individual perpetrators of crimes against humanity, war crimes and genocide. The US, which opposed its creation, has been determined to undermine it for fear that one day American troops might be in the dock. It refuses to hand over US nationals to the court and has signed bilateral immunity deals with several other countries guaranteeing that Americans would not be handed over to the court.

If the security council resolution is passed, as seemed likely yesterday, the prosecutors would begin a preliminary analysis to determine whether the crimes committed fall under their authority.

A report would be made to the security council in a few weeks on whether to launch a formal investigation.

The vote on a proposal made by the French was originally scheduled to take place last week, but was postponed to avoid embarrassing the US by forcing it to veto the resolution.

On Tuesday the 15 member security council did pass, by 12 votes to nil, a resolution strengthening an arms embargo on Sudan, with abstentions from Russia, China and Algeria.

The Sudanese government delivered a terse response, claiming there was "no justification" for the new measures, which also impose a travel ban on those considered responsible for the killings in Darfur as well as freezing any overseas assets they may possess.

Sudan's UN ambassador, Elfatih Erwa, blamed the resolution on a US Congress which "does not know the history, the culture of the people ... or does not understand it".