Markit Economics' flash manufacturing purchasing manager's index came in at 51, the lowest since October 2012.
But on a seasonally adjusted basis, that's the lowest reading since September 2009.
Economists had forecast that the preliminary reading will be 52.5, up just a bit from the prior reading of 52.4. But this reading is a big miss after an encouraging start to the year for the manufacturing sector, which has been challenged by the strong dollar and weak global demand.
"US factories are reporting the worst business conditions for over three years," wrote Markit chief economist Chris Williamson. "Every indicator from the flash PMI survey, from output, order books and exports to employment, inventories and prices, is flashing a warning light about the health of the manufacturing economy."
Diving into the details, manufacturing output growth slowed, while order backlogs were the weakest since September 2009.