If it follows through on its threat, the party will join other major opposition groups who have withdrawn candidates from the presidential race in protest against the vote they say is "rigged" in favour of Omar al-Bashir, the incumbent president.
The NEC's announcement came after Scott Gration, the US envoy to Sudan, held talks with election officials. On Friday, the Umma party said that Gration had agreed to seek a delay to the polls to ensure the widest possible participation.
But on Saturday Gration said that he supported the NEC's decision to go ahead with the polls.
"They [the NEC] have given me a lot of information that gives me confidence that the elections will start on time and that they will be as free and fair as possible," he said.
"This has been a difficult challenge but I believe they have stepped up and met the challenge."
Sudan's elections were thrown into crisis on Wednesday, when the Sudan People's Liberation Movement (SPLM) withdrew its presidential candidate, Yasir Arman, from the race after the ruling National Congress Party (NCP) rejected calls for the vote to be delayed until November.
Other opposition parties joined the boycott the next day, but divisions opened up between them over whether to pull out of legislative polls as well.
Al-Bashir wants to win the elections to legitimise his rule, but analysts say the credibility of the process will be deeply undermined if major opposition parties stay away from the vote.
But Ibrahim Ghandour, a senior NCP official, said the opposition threats would not affect the poll's legitimacy.
"If you know beforehand that this is a lost contest, why waste your time and resources on a lost bet?" he said. "This does not affect the legitimacy of the contest."
International observers and rights groups have said that all signs point to a flawed electoral process.
"The international community and the government of Sudan tried to build the elections like a house without foundation, and it is no surprise that [it] is falling apart pretty badly at this point," John Norris, the executive director of the Enough Project at the Centre for American Progress, said.
Al-Bashir, who seized power in a coup in 1989, has threatened to expel foreign election observers demanding electoral reforms.
Last week, he said that he would cancel a referendum scheduled for next January on southern independence if the SPLM boycott the elections.