The IMF's latest forecast, made in November, was for growth of 2.2%.
"It will be revised to 1 to 1.5% in 2009, which is huge," Axel Bertuch-Samuels, deputy director of IMF's monetary and capital markets department, told Reuters on the sidelines of a conference in the United Arab Emirates.
"Global economic prospects have deteriorated in recent months, consumer and business confidence have dropped to levels that we have not seen in decades and activity too has dropped sharply," he said.
The 2009 year will be enormously challenging for the world's economy, he said.
In November, the IMF cut projections sharply for world growth in 2009 to 2.2%, down 0.8 percentage points from an October forecast, noting industrialised economies were headed for the first full-year contraction since World War Two.
An official release of updated IMF economic forecasts is expected on Wednesday, he said, and even forecasts for emerging markets like China and India will see downward revisions.
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