In its final estimate for the month, Markit said the Purchasing Manager's Composite Output Index for the eurozone came to 46.7. An earlier projection, known as a flash estimate, placed it a 47.4.
The April figure is down from 49.1 in March.
Numbers below 50 indicate contraction and numbers above 50 indicate economic growth.
The drop was the steepest for the eurozone since October.
Markit said its monthly survey put the PMI for service businesses at 46.9, down from 49.2 in March.
"Contractions [were] seen in France, Italy and Spain while Germany slips closer to stagnation," the report said.
In Germany, the index came to 50.5, just above the break-even point. In Ireland, the PMI hit a three-month low of 51 and in France it was at a six-month low of 45.9.
Italy's PMI of 42.7 in April is a 36-month low. Spain's posting of 42 is a five-month low.
"The final Eurozone PMI came in well below the flash reading as business conditions deteriorated at a faster rate towards the end of the month. The survey suggests that the economy was contracting at a quarterly rate of around 0.5 percent in April, extending the downturn into a third successive quarter," said Markit Chief Economist Chris Williamson.
The numbers suggest that stimulus measures implemented by the European Central Bank "have not had a lasting impact on the real economy," Williamson said.