Lam Akol believes his party was leading in several southern states
The leader of Southern Sudan's second largest party has told the BBC there was "massive rigging" in Sudan's recent landmark elections.
Lam Akol, head of SPLM-Democratic Change, and the leaders of eight other southern parties have decided to challenge the result in the courts.
Salvir Kiir, leader of the Sudan People's Liberation Movement (SPLM), won 93% of the vote in the south.
His only challenger was Mr Akol - who is seen as the north's proxy candidate.
Mr Akol told the BBC's James Copnall in Khartoum that he wants the courts to declare the result in the south null and void because the election was fraudulent and the result did not reflect public opinion.
Omar al-Bashir, NCP: 68%
Yassir Arman, SPLM (withdrawn): 22%
Abdallah Deng Nhial, PCP: 4%
Hatim al-Sir, DUP: 2%
Sadiq al-Mahdi, Umma (withdrawn): 1%
Salva Kiir, SPLM: 93%
Lam Akol, SPLM-DC: 7%
Source: National Elections Commission
"The facts are so obvious," he said.
"A candidate [Mr Kiir] getting 93% of the vote is never heard of in a democratic process.
"It is also evidence that there is extensive rigging."
Mr Akol claimed that during the vote the SPLM's security forces took over polling stations, threatened and intimidated voters, and arrested opposition party agents.
He said his party was leading in a number of states in the south and that the results were doctored.
The polls were Sudan's first multi-party elections in 24 years and the first since the end of the 21-year conflict between the SPLM and the north.
Mr Akol split away from the SPLM last year to form his own party, accusing the party leaders of failing to improve the lives of south Sudanese in the four years since the end of the war.
Sudan's President Omar al-Bashir won the national presidential poll with 68% of the vote, despite facing war crimes charges over Darfur.
Observers and opposition parties have complained of fraud and - particularly in the south - of intimidation.