The opposition parties and the SPLM complain that the national electoral commission is under the control of the ruling National Congress Party and is therefore unable to organise free and fair elections.
A meeting between South Sudan’s President Salva Kiir, who is leader of the SPLM and First Vice President of Sudan, Sudanese President Omar al-Bashir and Second Vice President Ali Osman Taha was cancelled over the issue.
“If Bashir wants to blackmail the people of southern Sudan, that blackmail will be confronted... it will be a violation of the rights of the people of southern Sudan,” Pagan Amum, the secretary general of the SPLM, told RFI.
“Bashir can go ahead with these elections and he will contest them alone. At the presidency, no candidates will contest against him. He wants to be a president, so he can have it. He can have an easy walk to continuous presidency.”
Under a 2005 peace agreement that ended a 22-year civil war, the mostly-Christian South Sudan earned the right to hold a referendum on breaking away from the Muslim-majority north in January 2011.
But on Monday al-Beshir warned that this could change if the SPLM carries out its threat.
"If the SPLM boycott the elections, we will reject the referendum," Beshir said.
The polls, which are due to be held on 11 and 13 April, would be Sudan’s first multi-party elections in almost 25 years.