Opposition parties warn that the poll is unlikely to be either free or fair
One of Sudan's main opposition parties has said that it will boycott all the elections due to start on Sunday.
A spokesman said the Umma party would not take part in the presidential, parliamentary or state polls because of fears of rigging.
The poll had been billed as Sudan's first multi-party elections since 1986 but only minor opposition parties are now taking part in northern areas.
The main party of South Sudan, the SPLM, is only contesting in the south.
The BBC's James Copnall in Khartoum says the withdrawal of the Umma party from the elections will further damage the credibility of the democratic process.
It is one of the oldest and most popular parties in northern Sudan and won the 1986 polls, only to be ousted in a coup led by current President Omar al-Bashir.
"We have decided to boycott the electoral process at all levels," said Sarah Nugdalla, head of Umma's political bureau.
No access to Darfur
The Umma party was among a group of opposition parties that had given the Sudanese government a deadline to introduce reforms in return for a pledge to take part in the elections - as long as these were delayed until May.
By James Copnall BBC News, Khartoum
President Bashir is now all but assured of a resounding victory in the presidential elections - irrespective of how free and fair they are.
His main challengers will now be Hatem al-Sir of the DUP, and Abdallah Deng Nhial of the PCP. But neither man is even the head of his own party.
More generally, Umma's boycott strikes a further crushing blow to the credibility of these polls.
President Bashir will point to the participation of the DUP and the PCP, and suggest he has won a democratic mandate, which is particularly important since he was indicted by the International Criminal Court.
But any victory will certainly not have the force of a win against a full line-up of candidates.
Other smaller parties announced a full boycott of the election last week.
Earlier on Wednesday, the European Union announced that it was pulling its election observers out of the Darfur region over security fears.
"In some parts of Darfur the violence is terrible. The humanitarians cannot access this area. And if aid cannot access, we cannot access," said Veronique De Keyser, who heads the EU's election mission in the country.
She added that she feared the team would only be able to see a small portion of the voting, and could miss irregularities.
The president is hoping to win the polls to defy an International Criminal Court warrant over alleged war crimes in the Darfur region.
With no strong challenger, Mr Bashir, who seized power in a bloodless coup in 1989, looks set for a comfortable win, say analysts.
The separate north-south civil war ended in 2005, with a deal for the SPLM to share power with Mr Bashir's National Congress Party at the national level, while enjoying considerable autonomy in the south.
Some 1.5 million people died in the conflict between the mainly Muslim north and the south, where most people are Christian or follow traditional beliefs.
A referendum is due in 2011 on whether the south should secede and the SPLM is concentrating on staying in power in the south ahead of that vote.
Salva Kiir, the President of Southern Sudan and Vice-President of Sudan, explains his party's boycott of the forthcoming elections.
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