"We announce the SPLM (Sudan People's Liberation Movement) boycott of all the elections in the north on all levels ... in 13 states of the north," party secretary general Pagan Amum told reporters, saying Sudan's incumbent President Omar Hassan al-Bashir had rigged the vote.
Sudan's presidential, legislative and gubernatorial elections, due to start on Sunday, are central to a 2005 peace deal signed between the SPLM and Bashir's northern National Congress Party (NCP) ending more than two decades of civil war.
The SPLM's move will be seen as a rebuke to Bashir and will stoke tension as both sides prepare for an even more sensitive referendum on the independence of Sudan's oil-producing south, also promised under the peace deal, in January 2011.
Analysts say Bashir is hoping to show he can win a competitive election to legitimise his rule and fend off his indictment by the International Criminal Court to face charges of war crimes in Darfur.
Senior NCP official Ibrahim Ghandour told Reuters he was "disappointed" by the decision, but said it would not affect the legitimacy of the election, or plans for the referendum.
Amum told reporters the party was taking the decision in protest at widespread irregularities in the build-up to the vote. The SPLM and opposition parties have accused the NCP of clamping down on campaigning and rigging voter registration and other preparations. The NCP denies the accusations.
Amum said the decision excluded the border states of Blue Nile and South Kordofan, adding the SPLM would continue to run at all levels in south Sudan.
Tuesday's announcement marked an escalation in SPLM protests.
Last week the SPLM said it was pulling out just of presidential elections and all votes in the strife-torn Darfur region, leaving opposition groups in disarray over whether to follow suit.
Analysts had speculated the SPLM's partial boycott last week was part of a deal with the NCP to allow Bashir to win the elections in exchange for a guaranteed referendum. SPLM candidate Yasir Arman was seen as Bashir's main contender and his withdrawal increased the likelihood of an NCP victory.
But the SPLM's wider boycott on Tuesday suggested the party had dropped efforts to placate Bashir in the north.
There have been reports from party insiders of a split in the SPLM between mostly northern members keen to discredit the elections, and mostly southern members keen to preserve the referendum at all costs. Southerners are widely expected to vote for independence.
The SPLM joined a national coalition government with the NCP after the 2005 peace deal. But distrust remains deep and forces from both sides have clashed since the signing of the accord.
The US State Department on Monday said it was concerned about "serious restrictions on political freedoms" in the elections and said there might have to be a brief delay to the ballot to give authorities time to respond to complaints.
