Making the announcement on Friday, Zakim Shah, chairman of the UN-Afghan Electoral Management Body, also urged the international community to step up security assistance.
"These efforts will greatly enhance the legitimacy of the electoral process and the authority of the future assemblies," Shah said.
Afghanistan's interim-government had hoped to hold both elections in June, then September, but logistical problems and security worries forced delays.
Shad said the Electoral Management Body had considered delaying both polls but concluded that "the interests of stability would not have been served by further postponement."
"The interests of stability would not have been served by further postponement"
UN-Afghan Electoral Management Body
UN Secretary-General Kofi Annan has welcomed the timing of both polls.
His spokesman said the additional time before parliamentary polls was "a very valuable opportunity to create better conditions for the holding of a free and fair parliamentary election, which the government and the international community should not miss."
The Taliban and other armed groups ranged against the US-backed interim government have vowed to disrupt the polls.
In recent weeks, they have killed several election workers and 16 bus passengers found carrying voter registration card.
On Thursday, a landmine blast killed a woman working to register voters in the eastern province of Nangahar.
Karzai, installed after US-led forces drove out the Taliban from power in December 2001, is widely seen as presidential favourite. But at least a dozen rivals have indicated an intention to challenge him.
More than six million of an electorate of some 10 million have registered to vote.