President Karzai made his election date announcement on Saturday
Afghanistan's Election Commission has rejected President Hamid Karzai's call for an April presidential vote, saying it will take place on 20 August.
The head of the commission said it was not possible to hold the vote earlier.
Elections were due by May but had been postponed to August for security and logistical reasons - a delay that Mr Karzai challenged last weekend.
The UN and Nato welcomed the new ruling, saying the extra time was needed to improve security.
Mr Karzai had issued a decree at the weekend calling for the poll to be moved forward to April to comply with the constitution.
According to Article 61 of the constitution, elections should be held 30 to 60 days before 22 May, the end of Mr Karzai's five-year term. The president argued that meant the elections would have to be held no later than 21 April.
Mr Karzai's opponents, plus the US, Nato and the UN, have all been against the earlier vote.
A statement from Nato's International Security Assistance Force (Isaf) in Afghanistan said the commission's move would enable them to provide the "right security conditions" in support of the Afghan forces.
"While we respect and accept the president decree, its implementation is not possible because all the problems which we listed previously, which are mainly security and weather conditions, are still in place," the AFP news agency quoted Azizullah Ludin, the head of government-appointed Independent Election Commission (IEC), as saying.
"This is why after a thorough study the [commission] came to the conclusion that we have to hold transparent, free and fair elections. That's why we confirm the date of 20 August," he said.
The special representative of the UN Secretary-General for Afghanistan, Kai Eide, said in a statement: "This date will provide the time needed for the IEC with the support of the international community to make practical and logistical preparations, for candidates to campaign, and for Afghan and international forces to provide security."
Other international monitors have also said it would be difficult to hold a fair election by April for similar reasons.
The BBC's Martin Patience in Kabul says that what is not clear is what happens between the president stepping down and the August elections.
Mr Karzai wants to remain in power but his opponents say that he should be removed from office when his term comes to an end.
Our correspondent says that both the president and parliament will now need to reach an agreement to resolve this issue.
The US is also likely to support the commission's recommendation for an August poll, as the 17,000 foreign troops expected to bolster peacekeeping forces could be used to secure voting stations from the Taleban.