The prime minister has written to the other 24 members of the European Union to press for an end to violence in the Darfur region of western Sudan.
Tony Blair's letter is part of an effort to pressure Sudan's government to stop military action in the area.
Tens of thousands of people have died and up to two million have been displaced in three years of conflict.
His letter coincides with a Darfur Day of Action in 30 cities worldwide, including London.
Senior members of the Muslim, Jewish and Christian faiths have prayed for Darfur outside Downing Street.
A prayer from Archbishop Desmond Tutu - written with the Archbishop of Canterbury Dr Rowan Williams, Chief Rabbi Jonathan Sacks and Sheikh Ibrahim Mogra - remembered the dead and the displaced, those charged with trying to find a solution, and those perpetrating the violence.
"We pray that those who causing misery and death in Darfur will turn away from racism and violence - may they be forgiven when they turn to You instead."
The Darfur Peace Agreement, signed by the government and one of the major rebel groups in May, has failed to end fighting between rival rebel factions.
The mandate of African Union peacekeepers in the region expires at the end of the month and the Sudanese government has refused to approve their replacement by a UN force.
Mr Blair said the EU should play a "central role" in peace efforts and that the Sudanese government should prepare to face isolation if it failed to respond to diplomatic pressure to end the "slaughter".
"The EU should play a central role in mobilising world opinion on this issue," he wrote in the letter, which has also been sent to European Commission president Jose Manuel Barroso.
"We should strongly call upon the government of Sudan and non-signatories alike to stop immediately the violence in northern Darfur."
Baroness Amos, the Leader of the House of Lords, said that if the Sudanese government refused to allow UN peacekeepers into the region, the British government would press for further sanctions and for human rights abuses to be investigated by the international criminal court.
Up to two million people have been displaced in three years of conflict
"They have a responsibility to look after all of their citizens. We want that UN peacekeeping force in there. We can't see the kind of scale of devastation, human rights abuses, people being killed. The world community cannot stand back and watch this unfold yet again."
But Sudan's junior foreign minister, Ali Khati, said the situation on the ground was improving. He denied that his government had bombed villages.
And he was critical of the Western media's coverage of events in Sudan.
"People there in the West, in Europe and the United States, are moved by the media. And the media is unfortunately moved by political agendas, and by people making problems in the Third World, like Sudan."
The crisis began in 2003 when ethnic African tribes revolted against the Arab-led Khartoum government.
Actor George Clooney recently addressed the UN on the issue, and said that, if UN forces were not sent in, all aid workers would leave and the 2.5 million refugees who depend on them would die.
"The US has called it genocide. For you it's called ethnic cleansing. But make no mistake - it is the first genocide of the 21st Century."