The SPLM said it would only contest the election in two northern states
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.
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The SPLM has already withdrawn its candidate from the presidential poll.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.
The BBC's James Copnall in the capital, Khartoum, says it is a decision that will worry many people in Southern Sudan.
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.
The SPLM serves in a national coalition with President Bashir's National Congress Party (NCP), after joining a unity government in 2005, as part of a peace deal ending a two-decade civil war.
James Copnall, BBC News, Khartoum
The SPLM's decision to boycott all elections in the north is a gamble, and divided the party. The former southern rebels had already withdrawn from the presidential election and from polls in Darfur.
Many in the SPLM worried that by extending the boycott to most of the north, it would give President Bashir an excuse to reject the referendum on possible independence for the south, the SPLM's real priority.
But the northern SPLM politicians were determined not to run in protest at alleged rigging. Their future credibility was at stake - a particularly uncertain future since the bulk of their party would disappear if the south does become independent.
Ibrahim Ghandour, a senior figure in Mr Bashir's party, told the BBC the SPLM's partial boycott would not compromise the referendum. But with distrust between the south and north so great, many southerners will not believe his assurances.
The party said it would still contest the elections in its southern stronghold, as well as in Blue Nile and oil-rich South Kordofan - both northern states where people fought on behalf of SPLM against the Khartoum regime during the civil war.
The SPLM secretary general, Pagan Amum, said the partial boycott was to protest about insecurity in war-torn Darfur and alleged election rigging in the country as a whole.
"The SPLM will continue its leadership of the mass movement calling for free and fair elections, calling for the handing over of political power to the Sudanese people through free and fair elections," he said.
He added that he was dismayed by Mr Bashir's recent threats to election monitors who intervened in Sudan's affairs.
"We're entering into these elections and the National Congress Party continues to issue threats to observers.
"And you can imagine, if they are threatening observers to cut their limbs and noses what will happen to the Sudanese people?"
Some opposition parties have decided to partially or totally boycott the elections for the similar concerns about vote rigging, our reporter says.
Although the Democratic Unionist Party (DUP), which pulled out of the presidential race last week, has reversed its decision.
One major opposition party, Umma, has said it will decide on Wednesday whether to participate.
Veronique de Keyser, head of the European Union election monitors, has said she is dismayed by the lack of hospitality shown to observers, but she was determined to continue with their mission.
Earlier on Tuesday, Sudan's National Elections Commission insisted the vote would go ahead as planned on Sunday, Monday and Tuesday.
Africa's biggest country
Deeply divided along religious and ethnic lines
11 April elections intended to be first multi-party national poll for 24 years
Continuing conflict in Darfur
President Bashir wanted for war crimes in Darfur
South Sudan rebuilding after 21 years of civil war
South Sudan could secede in 2011
Large oil fields near north-south border
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.
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.