The eurozone's purchasing managers' index (PMI), compiled by data and research group Markit, slid to 49.
5 points in June, from 51.1 in May, according to an initial estimate.
The fall brought the index below the 50-point level indicating a contraction in activity for the first time since July 2003.
Meanwhile, the survey's component for the services sector fell to 49.
5 points in June from 50.6 in May, hitting the lowest level since June 2003.
On the manufacturing front the PMI index was also down, to 49.1 in June from 51.6 in May, the worst reading since May 2006.
Economists said that the fact that business activity shrank spelled bad news for the broader eurozone economy in the months ahead.
"The 'flash' June service sector and manufacturing purchasing managers' surveys for the eurozone make very worrying reading, showing a toxic cocktail of essentially stagnant economic activity but elevated and still rising inflation," said economist Howard Archer at consultants Global Insight.