NEW YORK (AP) _ Officials discovered high levels of lead in drinking water at a large government office building at least one month before informing anyone, according to a news report citing federal documents.

The story in Saturday's editions of The New York Times said the drinking water at the 42-story building in lower Manhattan was discovered to be tainted in early April.

But the 10,000 employees at 26 Federal Plaza, the second largest federal building in the nation after the Pentagon, were not told of the problem until May 13, the Times said.

On that date, a memo from the building manager said tests had found ''on a sporadic basis the safe level for lead is marginally exceeded.'' Those in the building were advised not to drink more than a quart of water a day from the building's water supply.

Experts say that ingested lead can be dangerous even in low concentrations, and particularly in fetal development.

''If they thought they had a problem, why didn't they inform us sooner?'' said Dr. Gerald Iwan, director of drinking-water quality control for the city Department of Environmental Protection.

''What shocked me,'' added Dr. Doris Goldberg, director of the bureau of child health for the city Health Department, ''was that a warning was given about something potentially serious and they left it to the public to monitor it themselves.''

But Paul Chistolini, assistant regional administrator for public buildings at the federal General Services Administration, said initial test results were unclear and that the water was believed to be safe.

To be cautious, however, Chistolini has ordered bottled water for the building at a cost of $35,000 a month. He also has commissioned more tests.

Blood tests of employees at the building are planned soon, the Public Health Service said.

The problem, the Times said, is believed to have been caused by pipes soldered with a lead compound commonly used in both public and private water- supply systems across the country. Some localities such as Portland, Ore., have banned the use of such compounds.

There also is a proposal in Congress to enact a nationwide ban.