(For more on Pakistan and Afghanistan, click [nAFPAK])

WASHINGTON, Oct 16 (Reuters) - An investigation into fraud in Afghanistan's election has cut President Hamid Karzai's vote tally to about 47 percent, a result that will trigger a runoff, the Washington Post reported on Friday.

Citing officials familiar with the results, the newspaper said the tally by the independent Electoral Complaints Commission was due to be finalized on Friday.

Preliminary election results issued on Tuesday gave Karzai more than 54 percent of valid votes tallied, putting him above the 50 percent threshold needed to avoid a runoff with his closest rival, former Foreign Minister Abdullah Abdullah.

But Afghanistan's ambassador in Washington, Said Jawad, said on Thursday that a second round of voting was "likely," the Post reported.

In a New York Times interview published on Thursday, the ambassador said Karzai's government was preparing for the electoral commission to announce on Saturday a runoff was necessary.

Citing a U.S. official in Afghanistan, the Post reported that ballots listing both Karzai and Abdullah Abdullah were printed in London in anticipation of a runoff and have arrived at the United Nations' mission in Kabul.

The inconclusive August election, where widespread fraud was reported, is a factor in the Obama administration's current review of its strategy in Afghanistan.

General Stanley McChrystal, the top U.S. and NATO commander in Afghanistan, has recommended deploying an additional 40,000 U.S. troops, beyond the 68,000 due to be in place by the end of this year.