(Reuters) - The International Monetary Fund on Monday cut its global growth forecast and warned that the outlook could dim further if policymakers in Europe do not act with enough force and speed to quell their region's debt crisis.

In a mid-year health check of the work economy, the IMF also cautioned the productive capacity in a number of emerging market economies, such as China, India and Brazil, may be lower than previously believed and future growth could disappoint.

The IMF shaved its 2013 forecast for global economic growth to 3.9 percent from the 4.1 percent it projected in April, trimming projections for most advanced and emerging economies. It left its 2012 forecast unchanged at 3.5 percent.

Downside risks to this weaker global outlook continue to loom large, the IMF said in an update of its World Economic Outlook. The most immediate risk is still that delayed or insufficient policy action will further escalate the euro area crisis.

The global lender said advanced economies would only grow 1.4 percent this year and 1.9 percent in 2013.

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