ALBANY — Governor Paterson yesterday prohibited all but essential hiring in a sternly worded directive to state agencies.

Mr. Paterson said that if his cost savings targets aren’t met, he will impose a hard hiring freeze and other measures on agencies. He says he is willing to withhold budgeted funds if an agency fails to meet his cost-saving targets.

The Democrat said the measures are necessary because of declining revenues projected in a worsening economy. Mr. Paterson required a detailed savings plan from each agency by May 16.

“The reductions you propose must be achievable, recurring, and serious,” Mr. Paterson said in the memo released yesterday. “Your plan must reflect the creativity needed to provide the services the public expects at a lower cost.

“Above all, you must rethink your hiring practices. Only job openings absolutely essential to your agency’s operations and protecting the health and safety of New Yorkers are to be filled,” he said. “Positions that do not fit this criterion must be left vacant. “

Mr. Paterson’s call shocked the state’s largest public employee union, which like other unions had planned on increases in hiring as part of the state budget approved earlier this month.

“We did not have any forewarning of this,” a spokesman from the Civil Service Employees Association union, Stephen Madarasz, said. “We’re extremely disappointed there was no discussion of this direction.”

He warned that Mr. Paterson’s plan could be wasteful, because the state has in many areas agencies have lacked enough staff in recent years to effectively do their jobs. For example, adding enforcement jobs can bring in more revenue from fines than the cost of the positions. Mr. Madarasz said the governor’s memo will also strike at employee morale.

“When you don’t give us some forewarning on this, it certainly undermines the confidence and trust in labor relations,” he said. The state work force stands at 167,172 employees, according to the state Department of Civil Service.