A federal judge blocked construction of a germ-warfare testing laboratory in Utah yesterday, saying that the Army had failed to prepare an adequate environmental assessment of its potential dangers.

The ruling, issued here by U.S. District Court Judge Joyce Hens Green, will hold up a planned $1.4 million "maximum containment" chamber at the Dugway Proving Grounds. Congress approved the chamber, which will be used to test airborne toxins, last year as part of a major program to modernize the base.

Green rejected the argument of the plantiffs, the Foundation on Economic Trends, that the new laboratory would "inevitably be used to conduct tests involving genetic engineering," despite Army assurances to the contrary.

But she called the laboratory a "major federal action" and said the 20-page environmental assessment that the Army filed in January was "clearly inadquate" under standards the U.S. Court of Appeals here established for applying the National Environmental Policy Act.

"The possibility of an accident involving personnel or exposure to the outside environment, while low in probability, does exist," Green declared. "Clearly the risks are serious and far-reaching. Such an accident could produce extraordinary, potentially irreparable, consequences."

The base is 95 miles southwest of Salt Lake City and 60 miles from the nearest town.

An Army spokesman said the decision "will delay still further an urgent defense project." He said the Army had not decided whether to appeal or take "some other course of action" that would "most quickly remove the present restraints."

Jeremy Rifkin, the foundation's president, said he was "very pleased" with the ruling.

"We're not going to quibble," he said, referring to Green's failure to accept all of the group's arguments. "We're happy we won."

The Army has operated Dugway as a chemical- and biological-warfare testing station since World War II. It told the court that the new laboratory, built to the most stringent safety standards set by the National Institutes of Health, would improve protection against the escape of microbes already being studied at Dugway.

The Army said the laboratory would be used to test the effectiveness of defensive equipment, such as gas masks, boots, gloves and alarms, and would not produce or develop offensive weapons.

If the Army decides to use the laboratory to test more dangerous microbes, it promised to prepare an "appropriate environmental impact analysis" then. But it told the court it wanted to build the chamber now to avoid a "devastating delay" in case it found out about more dangerous Soviet germ weapons.

The Army said Dugway was the only facility that could be used to test germ-warfare defenses although four laboratories around the country meet the stringent NIH standards, including one at Fort Detrick in Frederick, Md.

In ruling yesterday, Green said she was certain that any delay "would be insubstantial" because an environmental assessment could be less detailed than a formal environmental impact statement. But she said the assessment the Army had filed was only "an amalgam of conclusory statements and unsupported assertions of 'no impact.' "

The ruling allows the Army to begin construction of nine other facilities at Dugway, costing $7 million, that the suit had also challenged. Besides Rifkin's group, plaintiffs include retired Adm. Gene R. LaRocque, director of the Center for Defense Information, and a retired Marine Corps major general, William T. Fairbourn, of Salt Lake City.