"If there is an election, it will be a disaster for Sudan," the AFP news agency quoted Faruk Abu Issa, a spokesman for the coalition of opposition groups, as saying.
Opposition presidential candidates are due to meet on Thursday to make a final decision on the proposed boycott.
'Chaos and war'
Khalil Ibrahim, the leader of the Justice and Equality Movement (Jem), the biggest rebel group in Sudan's western Darfur region, joined the call on Wednesday for a delay in the election.
"These elections are based mainly on false senses, especially in Darfur. Masses of populations ... will be excluded from the elections," he told Al Jazeera.
"Especially the nomads - they can not participate in these elections. So we are calling for the delaying of this and we want to accelerate the peace process first."
Jem has been negotiating a peace deal with the Sudanese government, but Ibrahim said it would be a catastrophe if Omar al-Bashir, the country's president, wins the election.
"He will continue the violence, especially in the west part of Sudan," he said.
"I don't think the other parties will accept this, there will be chaos and war if he [al-Bashir] wins."
Al-Bashir has rejected calls to delay the vote, saying he will refuse to hold a referendum on the autonomy of the country's south if the Sudan People's Liberation Movement (SPLM) boycotts the forthcoming elections.
Al-Bashir's National Congress Party (NCP) has ruled Sudan in a coalition with the SPLM since a peace deal that ended war between the two sides of Sudan in 2005.
"Holding elections in the Sudan is a national obligation that should be fulfilled," Al-Bashir, who has ruled Sudan for more than 20 years without holding elections, said on Monday.
"They [the SPLM] are calling for cancelling the elections and holding the referendum. This is unacceptable nonsense.
The presidential and legislative elections on April 11 will be the country's first multiparty polls in 24 years.
Yan Matthew, a spokesman for the SPLM, said the referendum would go ahead and that his party was only insisting on free and fair elections.
"The  agreement did not speak about the right of the partners to postpone the referendum in the South ... There are explicit items in this regard as in articles 122 and 123 of the peace agreement," Matthew told Al Jazeera.
"What has happened is an manifestation of anger [by the NCP] after they knew that the SPLM is the real threat to the power they had grabbed by force from the Sudanese people."
The opposition wants the polls delayed until November, citing a continued conflict in Darfur and unresolved complaints of electoral irregularities.
The north-south civil war that began in 1983 claimed an estimated two million lives and destabilised much of east Africa.