"This is why after a thorough study the EC came to the conclusion that we have to hold transparent, free and fair elections. That's why we confirm the date of August 20."
There are fears that the election could be plagued by violence as the Taliban, which has said it will not take part in the process, is gaining strength in large areas of the country.
About 17,000 additional US troops being sent to Afghanistan will be in place for the August vote, but would not have been available to help provide security for a spring election.
In a sign of the challenges faced in Afghanistan, a suicide bomber detonated his explosives outside an air base used by US forces on Wednesday.
A number of people were injured in the blast at the Bagram air base, about 60km north of the capital, Kabul, US military officials said.
Abdul Jabar Takwa, the governor of Parwan province, said that the bomber was in a car that exploded after he drove past a police checkpoint.
A Taliban spokesman told Al Jazeera that it carried out the attack.
Afghan opposition groups and rival presidential contenders, as well as the United States, had all backed the original August date.
But with that date being confirmed, it is now unclear who will be president after Karzai's term ends.
Al Jazeera's James Bays, reporting from Kabul, said: "Hamid Karzai would like to stay on as president, but those who are planning to challenge him for the job say that is unfair.
"They say if Karzai was the interim president ... he would have all the advantages of the presidency in this war-ravaged country.
"He would have security around him, the ability to fly anywhere he wanted by helicopter, giving him an unfair advantage in this election."
The National Front, the main political opposition group formed of a fragile coalition of opposition parties and some other candidates welcomed the commission's decision and called for "national dialogue" on an interim administration.
Separately, the Nato-led International Security Assistance Force (Isaf), which is fighting alongside US forces in Afghanistan, said that three of its soldiers had died in a roadside blast in the south of the country.
A Nato statement said that the troops were killed in the blast on Tuesday, but did not release the exact location of the attack.
Brigadier-General Jon Vance, the commander of Canadian troops in Afghanistan, told the Canadian press that the three were Canadian.