South Sudan's main party has said it will boycott this week's elections in most of northern Sudan, citing voting irregularities and security fears, BBC reported.
The Sudan People's Liberation Movement (SPLM) said it would not participate in the parliamentary and municipal polls in 13 of the 15 northern states.
The SPLM has already withdrawn its candidate from the presidential poll.
The elections beginning on Sunday are supposed to be Sudan's first multi-party vote since 1986.
President Omar al-Bashir has threatened to cancel an independence referendum due in January for the already semi-autonomous south, should the SPLM boycott the poll.
'No delay'
The SPLM serves in a national coalition with President Bashir, after joining a unity government in 2005, as part of a peace deal ending a two-decade civil war.
The party said it would still contest the elections in its southern stronghold, as well as in the northern states of Blue Nile and South Kordofan.
The SPLM secretary general, Pagan Amum, said the partial boycott was to protest about insecurity in war-torn Darfur and alleged election rigging.
Many opposition parties have decided to partially or totally boycott the elections for the same reasons, says the BBC's James Copnall in the Sudanese capital Khartoum.
One major opposition party, Umma, is still deciding whether to participate.
Earlier on Tuesday, Sudan's National Elections Commission insisted the vote would go ahead as planned on Sunday, Monday and Tuesday.
There would be "no delay," Hadi Mohammed, a senior official in the body, told reporters after following talks with US special envoy to Sudan, Scott Gration.
The ruling National Congress Party (NCP) of President Bashir, who is wanted for alleged war crimes in Darfur, insists the elections will be fair.
"When they realised that the NCP was going to win, they decided to say that there is fraud," Mr Bashir told a rally on Tuesday, reports AFP news agency.
Last week, the SPLM announced the withdrawal of its presidential candidate Yassir Arman, a key challenger to President Bashir.
With no strong challenger, Mr Bashir, who seized power in a bloodless coup in 1989, looks set for a comfortable win, say analysts.
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.