The Advertising Standards Authority upheld a complaint from pro-Sudanese government group. the European Sudanese Public Affairs Council, against the campaigning group Globe for Darfur.
Globe for Darfur claimed in a national press ad that 400,000 people have been killed in three years in the Darfur region by Janjaweed Arabic rebels, who are backed by the government in Khartoum.
The complaint from the European Sudanese Public Affairs Council, which challenged the claim about the number of people killed, was the only one received by the advertising watchdog.
The ad stated: "In 2003, Sudanese President Omar al-Bashir moved to crush opposition by unleashing vicious armed militias to slaughter entire villages of his own citizens. After three years, 400,000 innocent men, women and children have been killed..."
Save Darfur Coalition and the Aegis Trust provided a submission to support the use of the claim, which explained the background to the Darfur genocide and how both organisations became involved in trying to raise awareness of the Darfur crisis.
They said they had based the claim of 400,000 deaths on a report issued by the Coalition for International Justice in April 2005, which was based on interviews conducted by the CIJ in Chad refugee camps in 2003.
They also provided a statement from Dr Hagan, Professor of Sociology and Law at Northwestern University, which explained that a death toll of 400,000 was "within the realms of possibility".
Save Darfur Coalition and the Aegis Trust admitted that the US General Accountability Office concluded from its study of six mortality studies, including the CIJ report, that none could be considered to have a "high level of accuracy".
The European Sudanese Public Affairs Council pointed out that GAO's report had considered the Centre for Research on the Epidemiology of Disasters report to be the most objective, which estimated 120,000 deaths between September 2003 and January 2005.
The ASA upheld the complaint and told Save Darfur Coalition and the Aegis Trust to present the figure as opinion not fact in future and advised them to state the source of such claims.
The watchdog concluded that there was a division of informed opinion about the accuracy of the figure contained in the ad and that it should not have been presented in such a definitive way.