"After a thorough review, it is clear that no candidate has been able to win more than 50 per cent and the election goes to a second round," Ahmad Yusuf Nuristani, head of the Independent Election Commission (IEC), said.
Abdullah secured 45 per cent of the vote on April 5, with his main rival Ghani on 31.6 per cent, according to the final results, which came after weeks of deliberation over fraud allegations.
The results were closely in line with the figures released late last month when counting was completed. The run-off was originally scheduled for May 28.
"Some sensitive materials that were stocked at IEC headquarters for the second round were destroyed by the Taliban attack on March 29 -- providing those materials again needs time," Nuristani said, explaining the delay.