Angola opposition party National Union for the Total Independence of Angola (UNITA), said it would file a complaint that would have the effect of suspending the declaration of results of the vote.
UNITA's letter says the party's representative at the commission "was not granted the right to record in the result sheet his complaint about the electoral results".
UNITA leader Adalberto Costa Junior has repeatedly said in the past few days that he does not recognize the results of the vote and that complaints have been filed.
"UNITA reiterates that it will not recognize the results announced by the National Electoral Commission until the complaints already in its possession are resolved," the party said in its Tuesday statement.
It vowed to file a legal claim "which will have the effect of suspending the declaration of the final results", the party's secretary general, Alvaro Chikwamanga Daniel, said in a video recorded overnight.
During the final phase of counting, the former rebel movement-turned-opposition-party claims "not to have been informed of the decision" by the electoral commission to ratify the results and not to have received a "copy of the tables of the final results."
Under Angola's laws, if UNITA wishes to challenge the results it must first lodge a complaint with the commission. If that is rejected, the party can take the matter to the Constitutional Court, which must rule within 72 hours
After the most closely fought election since independence from Portugal in 1975, the commission declared the ruling People's Movement for the Liberation of Angola (MPLA) the winner, prolonging its nearly five decades of uninterrupted rule and handing President Joao Lourenco a second term.
Just over 51% of voters had supported the ex-Marxist MPLA, it said. UNITA, its longtime opponent and former civil war enemy, took about 44%, its best result ever, according to the commission.
Angola's Constitutional Court is led by Laurinda Cardoso, a former MPLA member who was appointed by Lourenco in August 2021. Analysts say the MPLA controls the court but Lourenco says it exercises its powers independently.
Angolan political analyst Edmilson Angelo said UNITA was likely to eventually accept defeat but would continue to reject the gap in votes.
"This will allow them to be coherent with their position on the electoral process while at the same time soften the feeling of disruption which may erupt," Angelo said.
Analysts say any dispute could ignite street protests but that has not happened so far.
Lourenco, 68, has pledged to press on with reforms in his second term, including privatizing poorly run state assets and continuing to clean up corruption after investigating wealthy and powerful members of the Dos Santos family.
Lourenco's reforms have so far failed to create a fairer distribution of Angola's vast oil wealth - it is Africa's second largest producer - which remains mostly in the hands of a few well-connected MPLA officials.
The MPLA and UNITA, formerly anti-colonial guerrilla groups, were on opposing sides of the post-independence civil war that lasted until 2002, when Angolan troops killed UNITA's rebel leader, Jonas Savimbi.
The MPLA's Jose Eduardo dos Santos, who died in Spain in July, had ruled Angola from 1979 until 2017, when he handed power to Lourenco.