A federal judge has ordered the Interior Department to restore wilderness protection to about 1.5 million acres of land in 11 western states, saying that James G. Watt, when he was secretary of the interior, illegally opened the lands to development.

In an opinion issued Thursday in Sacramento, U.S. District Court Judge Lawrence Karlton said Watt "in large measure . . . failed to follow the law" when he removed the acreage from wilderness study in 1982 and 1983.

Environmentalists hailed the decision, which overturns what one called "one of Watt's most capricious acts."

"It was just an attack on wilderness for no particular reason," Sierra Club spokesman Debbie Sease said.

But, because the decision is based largely on procedural grounds, she said, some of the tracts still could be removed from wilderness study.

Interior officials said the department has not reviewed the judge's ruling and that no decisions have been made on whether to pursue the matter.

Watt removed the lands from congressional review as wilderness areas, citing his discretionary authority to exclude tracts of less than 5,000 acres, tracts adjacent to proposed or designated wilderness areas and "split-estate" tracts, where the U.S. government owns surface rights and states or private parties own mineral rights.

Karlton ruled that Watt exceeded his authority when he excluded split-estate lands.

On the other tracts, he said, Watt violated administrative procedures, although he had "very broad authority" to exclude the lands.