The eurozone's purchasing managers' index (PMI), compiled by data and research group Markit, slid to 47.0 points in September from 48.2 in August, according to an initial estimate.
The new figure, which brought the index to its lowest level since November 2001, fell short of an economists' estimate of 47.
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"The fall in oil prices and the decline in the euro from mid-July to mid-September has done nothing to raise the spirits of the business sector in the eurozone," said Bank of America economist Holger Schmieding.
"To the contrary, the mood took a further sharp turn for the worse in September according to the flash purchasing managers indices," he added.
The index's fall marked the fourth consecutive month of contraction, which is indicated by a reading of less than 50 points.
Meanwhile, the index for activity in the eurozone's vast services secto