The ex-general who lost Indonesia's presidential election to Joko Widodo will challenge the result in court, his campaign team has said.
The move by Prabowo Subianto, announced on Wednesday, could spell weeks of uncertainty for the country.
Widodo, the governor of Jakarta, is seen as a break from the autocratic era of President Suharto. He was named the winner of the election on Tuesday, with results showing he had defeated his rival by about 8.4 million votes.
Before the result was announced in the world's third biggest democracy, Prabowo, who had also claimed victory in the July 9 vote, announced he was withdrawing from the election race.
He claimed Widodo's side had tampered with the votes during the long counting process.
Members of his campaign team indicated on Tuesday that he would not challenge the result in the Constitutional Court, as he had been widely expected to do.
On Wednesday, however, a spokesman for Prabowo, a top general in the Suharto era who has been dogged by allegations of human rights abuses, said he would contest the result.
"We are in the process of preparing our challenge to the Constitutional Court," said spokesman Tantowi Yahya.
Prabowo's brother Hashim Djojohadikusumo added: "We are looking for justice... We are expecting some fairness."
He also urged foreign leaders not to congratulate Widodo, as "the legal process has not ended yet".
US Secretary of State John Kerry and Australian Prime Minister Tony Abbott are among those who have already sent congratulations.
A spokesman for Widodo's team, Anies Baswedan, suggested the court may not accept the challenge.
"The court has always been selective in accepting cases," he told AFP news agency.
"Only cases that have merit and the possibility of changing results will be welcome".
Widodo's victory capped a meteoric rise for the former furniture exporter who was born in a riverbank slum, and won fans with his common touch during his time as Jakarta governor