WASHINGTON (AP) _ More than a year before the June 9 failure of feedwater pumps an Ohio nuclear plant, government regulators identified the system as the biggest potential source of a ''core-damage'' accident, according to documents released Sunday.

In a letter to the House Energy conservation and power subcommittee, the Nuclear Regulatory Commission said ''is not aware of any other'' plant safety system as vulnerable to failure as the feedwater system at the Davis-Besse plant near Toledo.

The NRC's July 24 letter consisted of responses to questions submitted by the subcommittee's chairman, Rep. Edward J. Markey, D-Mass.

Markey, an avowed opponent of nuclear power, accused the commission of ''voodoo regulation'' by not forcing the northwestern Ohio plant to immediately correct the problem after it was identified in a detailed study of possible accident scenarios.

That study was completed by the NRC in January 1984.

''The risk of a core-melt accident at Davis-Besse is more than five times the degree of risk that NRC has established as a safety goal,'' said Markey. ''But the NRC has never acted on this knowledge, an indication that in this case it abdicated its mandate to protect public health and safety.''

Unlike the 1979 core-damage accident at the Three Mile Island plant near Harrisburg, Pa. - the worst in the industry's history -operators at Davis- Besse were able to restart the pumps within 12 minutes after they failed and keep the reactor safely covered with cooling water.

The plant has been shut down since the mishap.

Harold Denton, NRC's director of nuclear reactor regulation, said Davis- Besse is the only plant without an electric-driven auxiliary feedwater system. Denton said the lack of such a backup is a major factor in the high probability estimate the plant received. The NRC has been pushing but not actually requiring the Toledo Edison Co. to install the system, he added.

''We do think it is important,'' said Denton. ''Davis-Besse is sort of the last one, the laggard.''

Toledo Edison spokesman Richard Wilkins said the utility ordered the third auxiliary pump last fall and that company officials have decided to expedite its installation.

''If that third feedwater system is the major factor, in the near future it won't be,'' said Wilkins.

The NRC letter indicated that the chances of core damage at Davis-Besse due to the loss of main feedwater were about one in 2,000. The commission noted that although there have been instances of accident sequences with greater probabilities, ''none remain so high today.''

However, Denton said Toledo Edison had conducted its own probability analysis, which had determined the risk of core damage due to loss of main feedwater to be less than as estimated by the NRC.

In the letter to Markey, Commissioner James Asselstine provided NRC estimates of core meltdown frequencies due to the loss of main feedwater at 10 plants, including Davis-Besse.

''I find those estimates, particularly when one considers the uncertainties, to be unacceptably high for the remaining life of the plants,'' Asselstine said.

Toledo Edison had originally proposed other steps to improve reliability of the auxiliary feedwater system but had later agreed to install the electric- driven backup pump.

''The years that it has taken to resolve the issues surrounding the upgrade of the auxiliary feedwater system at Davis-Besse is unusual and undesirable,'' the NRC noted in its letter.

Denton said there are discussions under way that could result in installation of the system before the plant is allowed to restart, possibly sometime this fall. The NRC estimated it will cost $1.8 million to install the third auxiliary feedwater system.

In the letter to Markey, NRC Chairman Nunzio Palladino said Davis-Besse won't be allowed to restart until the commission is satisfied that the plant can be operated safely, a finding that will be based in part on the effectiveness of improvements to the backup feedwater system.