Victims and witnesses are routinely threatened and sometimes even charged with crimes if they come forward with allegations of rape, the report released on Friday said.
"Many women do not report incidents, out of fear of reprisals," the report said. "Some police stations refuse to register and investigate complaints of sexual violence."
The report was drawn up at the request of Louise Arbour, the UN High Commissioner for Human Rights.
She wanted to assess Sudan's compliance with promises it made last year to punish those who commit sexual violence against civilians in Darfur.
It said the government has taken small steps - such as creating a committee on gender-based violence - but hasn't brought offenders to justice on a large scale.
"Basically we see extremely poor results in the investigation and successful prosecution of perpetrators of very serious sexual violence," Arbour told a news conference. "Rape, gang rape and so on continue to be very prevalent by all accounts."
The report said offenders include members of the law enforcement, security forces and pro-government militia, the report noted. Suspects have been tried very rarely, Arbour said.
"We're talking of a handful of cases where we've been able to document a successful prosecution," she said.
"We're talking of a handful of cases where we've been able to document a successful prosecution"
UN High Commissioner for Human Rights
Even when women do report sexual offences, authorities "generally deny the allegations of rape and sexual violence and intimidate the victims and witnesses into withdrawing their charges," the report noted.
In her news conference on Friday, Arbour said Sudan's government had been "extremely unreceptive" about recognising the magnitude of the problem.
"The government has to acknowledge that this is not a fabrication by humanitarian workers and NGOs and so on," Arbour said, referring to non-governmental organisations who have reported on widespread rape in Darfur.
Women are "preyed upon continually by members of the armed forces, by militias, by soldiers, policemen, people who should be brought under control by the government," Arbour said.
On Thursday, she told the UN Security Council that Sudan either can't or won't hold rapists accountable.
Earlier this year, the Security Council referred cases of crimes against humanity in Darfur to the International Criminal Court, the world's first permanent war crimes tribunal.
Lead prosecutor Luis Moreno Ocampo suggested in June that rape would be one crime he would pursue.