"There's all this talk of UN resolutions, but there's a failure to act, to give a big enough force powers to stop the killing," said Labour MP Clare Short.

"The UK should immediately be calling for a UN mandate under chapter seven (of the UN charter, allowing UN-approved military intervention without the consent of a state) so that the African Union force can be much longer, much larger," Short, a former development minister, said.

"We're focussing on that, not because the other things don't need doing, but let's do that first, and urgently."

The Protect Darfur campaign launch came as the Security Council in New York prepared for a controversial vote on whether to refer war crimes in the vast western Sudanese region to the International Criminal Court (ICC).

The council on Tuesday authorised sanctions, in the form of travel bans and asset seizures, against individuals who commit atrocities or undermine peace efforts in Darfur.

A parliamentary committee also advanced a new death toll this week for Darfur, saying more than 300,000 people had died in the region's 22-month conflict, begun when a local rebellion launched in February 2003 was put down by the Khartoum government with the help of Arab militias.

The militias and troops are accused of gross human rights violations, including widespread killing and rape in an ethnic cleansing campaign that has displaced more than 2.4 million people.

In this context, Conservative MP John Bercow said, the UN debate about sanctions against and ICC prosecution of the authors of the atrocities was "breathtakingly secondary to the enforcement of peace".

Bercow, a member of the international development committee which released the new death toll, dismissed the UN sanctions as powerless to convince Khartoum to rein in the atrocities.

"Does anybody really think the government of Sudan... is to be dissuaded from that massive premeditated military exercise by the thought that some members of the regime will be subject to a travel ban?" he said.

"Does anyone seriously think that they're going to be quaking in their boots at the thought of an asset ban?"