Full results from the April 5 election were published late last month, but the final declaration will factor in the outcome of weeks of deliberation over fraud allegations.
In the preliminary results, none of the eight candidates gained more than 50 percent of the vote, pointing to a second-round run-off between the two top names as Afghanistan undergoes its first democratic transfer of power.
The head-to-head contest would pit former foreign minister Abdullah Abdullah, who took 44.9 percent of the first-round vote, against ex-World Bank economist Ashraf Ghani, on 31.5 percent.
The Independent Election Commission (IEC) said it would release the results at 11:00 am (0630 GMT), and also confirm the date of the run-off, which is pencilled in for June 14.
Karzai, who has ruled since the Taliban were ousted in 2001, is constitutionally barred from a third term in office, and the next president will lead Afghanistan as 51,000 US-led combat troops withdraw by the end of the year.
Abdullah said on Wednesday that his campaign had evidence of fraudulent voting that could have a "significant impact on final results".
Abdullah this week received a major boost with the endorsement of third-placed Zalmai Rassoul, a close ally of Karzai, who has stayed publicly neutral in the election.
Another costly, and potentially violent, election could be avoided by deal-making in the coming weeks -- and Rassoul's support for Abdullah increased pressure on Ghani to concede.
The United Nations' mission has welcomed Afghanistan's conduct of the vote but warned officials that they must address all fraud allegations openly.
The 2009 election, when Karzai retained power after defeating Abdullah, was marred by ballot-box stuffing in a chaotic process that shook the multinational effort to develop the country after the Taliban's 1996-2001 regime.
The first-round election last month was hailed as a success, with turnout far better than in 2009 and the Taliban failing to launch a major attack despite threats to disrupt the vote.
- Fears for the future -
Both candidates have pledged to explore peace talks with the Taliban and sign a deal with Washington that could allow up to 10,000 US troops to stay on after this year on a training and counter-terrorism mission.
Many observers fear that Afghanistan faces a huge challenge suppressing the Taliban insurgency with declining foreign military assistance and also that the nation's economy could collapse as donor money falls.
An International Crisis Group (ICG) report released on Monday concluded that "the overall trend is one of escalating violence and insurgent attacks" in Afghanistan.
The insurgents "are blocking roads, capturing rural territory and trying to overwhelm district administration centres," it said.
"With less risk of attack from international forces, they are massing bigger groups of fighters and getting into an increasing number of face-to-face ground engagements with Afghan security personnel."
John Sopko, head of the US watchdog on Afghanistan's reconstruction, warned Wednesday that its aid-dependent economy may not be able to keep open infrastructure such as power plants that have been built with donor funds.
"Each new development project that the US and other international donors fund increases overall operation and maintenance costs, adding pressure to Afghanistan's operating budget," he told an audience in Washington.
"It is questionable whether the Afghan entities charged with financing these projects can afford them."