The EU said Thursday that next week's Sudan elections, widely expected to see President Omar al-Bashir extend his 25-year rule, cannot produce a "credible" result.
Bashir faces 15 little-known challengers while the main opposition parties are boycotting the vote in an impoverished country riven by deep political, religious and tribal divisions.
EU foreign affairs head Federica Mogherini said Bashir's failure to establish a genuine national dialogue, which he announced early last year, was a real setback and effectively undercut the polls.
"When dialogue is bypassed, some groups are excluded and civil and political rights are infringed, the upcoming elections cannot produce a credible result with legitimacy throughout the country," Mogherini said in a statement.
"The people of Sudan deserve better. We therefore chose not to engage in support of these elections."
The European Union would however continue to support the efforts of the African Union and others to bring peace to Sudan, she added.
At an October summit, EU leaders called on Bashir to seek a comprehensive solution to Sudan's problems, pressing him to include representatives from the country's many armed groups, the opposition and civil society.
Rights groups have accused 71-year-old Bashir of crushing dissent with a crackdown on the media and civil society.
A soldier, Bashir took power in an Islamist-backed takeover in 1989 as the country descended into a 22-year civil war which saw South Sudan split away.
A rebellion in the western Darfur region erupted in 2003, costing some 300,000 dead, and Bashir is wanted by the International Criminal Court for alleged war crimes in the region.
Insurgencies have also broken out in the South Kordofan and Blue Nile regions, led by former allies of south Sudanese rebel forces, and the Sudan People's Liberation Army-North has vowed to stop voting across the whole of the two regions.