An investigation of allegedly fraudulent ballots in Afghanistan's troubled election has reduced President Hamid Karzai's portion of the vote to about 47 percent, an outcome that will trigger a runoff between him and his closest competitor, according to officials familiar with results.

The tally by the U.N.-backed Electoral Complaints Commission, which one official called "stunning," is due to be finalized Friday. Preliminary results by Afghanistan's Independent Election Commission had given Karzai 54.6 percent of the Aug. 20 vote.