A comprehensive plan for controlling workers exposure to cancer-causing substances was proposed by the Labor Department yesterday in a major depature from its previous case-by-case approach to regulating health hazards on the job.

In announcing the policy, Labor Secretary Ray Marshall conceded the futility of existing regulatory procedures, under which the department's Occupational Safety and Health Administration has adopted standards for only 17 of nearly 2,000 suspected cartinogens in its six years of existence.

"Trying to control carcinogenic substances on a case-by-case basis is like trying to put out a forest fire one tree at a time," said Marshall. "Instead we are proposing a systematic way of determining which toxic substances require emergency attention by OSHA . . . It will allow us to respond to threats to worker health with much greater speed and efficiency."

The policy is not expected to take effect for at least a year but was hailed by Marshall as a "major break-through" in governmental efforts to curb cancer and by OSHA officials as the controversial agency's major achievement to date.

Announment of the plan coincided with disclosure of a National Institute of Occupational Safety and Health report estimating that nearly 1 million Americans may be exposed to cancer-causing substances at work. The report also said 20 million workers - about one in four - encounter substances that can cause disease or death.

The new OSHA cancer regulatory policy sets out procedures for classifying potential cancer-causing substances in one of four catagories confirmed carcinoznes suspected carcinogens, substances for which evidence is inconclusive and substances not found in American industry. Varying degrees of regulatory action would be triggered automatically for the first two categories.

In the case of substances found by tests to cause cancer in humansor two other species of mammals, an emergency order would be issued limiting worker exposure to the lowest levels feasible under current technology while a permanent regulation is prepared and submitted to a public hearing. Such a substance could be banned outright if there are subsitutes for it. At least 100 chemicals have already been identified as carcinogens and would be targets of immediate action, OSHA officials said.

Suspected but unproven carcinogens would be subject only to permanent orders, developed after public hearings. Substances in other categories would be listed publicly to alert workers of potential dangers.

The plan also contains model "fill-in-the-blanks" exposure standards for suspected carcinogens, aimed both at saving time and making standards more uniform.

The plan itself will not take effect until after public hearings scheduled for next March, Eula Bingham, administrator for OSHA, said the agency hopes to have all proven carcinogens under regulation within "a couple of years" after the policy becomes effective but noted that the timetable could be delayed by litigation.

Officials expressed hope that any litigation over the policy may establish legal precedents that will reduce time-consuming court challenges that have delayed imposition of some existing standards, such as the recent emergency order for benzene.

Immediate reaction to the new cancer policy was mixed, as expected. It was hailed by several labor groups and by Ralph Nader's Health Research Group but ran into opposition from the Society of the Plastics Industry, Inc., which called it a "quicky-fix method of dealing with the admittedly difficult task of reducing the risks of occupational health hazards."