Virginia Gov. John N. Dalton used his emergency powers tonight to restore a ban on fishing in portions of the James River tainted by the pesticide Kepone in an effort to prevent any contaminated fish from reaching the marketplace.

A state court judge had lifted the four-year-old ban a day earlier, saying the state had made procedural errors in extending it early this year.

Before Dalton acted tonight, marina owners who brought the lawsuit that temporarily overturned the ban said at least a dozen commercial fishermen had returned to the once-forbidden waters.

It was unclear tonight whether any fish or shellfish taken today by those fishermen would reach market.

Besides being a highly toxic chemical, Kepone is suspected by federal researchers of being a carcinogen because it has caused liver cancer in laboratory animals. State medical researchers say the pesticide's long-term health effects on humans are unclear.

Dalton said he acted because the Virginia Supreme Court could not hear for a week the state's request for a stay of the lower court's order lifting the ban.

"The watermen have returned to the river with no restrictions as a result of the judge's order, which increases the chances that fish or shellfish contaminated with Kepone will reach the marketplace," Dalton said.

The governor said he acted on the advice of Attorney General J. Marshall Coleman that he has separate powers under the state Emergency Services Act to use his judgement in the case of natural or manmade disasters.

The fishing ban has been imposed by executive orders of the state's governors since December 1975 when a federal research team found evidence that fish taken from the river had dangerous concentrations of the pesticide.

Kepone, used largely for dusting banana and European potato crops, had been secretly discharged into the river for years by two chemical companies that manufactured the chemical in Hopewell, a small town southwest of Richmond.

State health officials said today that fish from James River waters below Hopewell are likely to contain Kepone levels at least as high as allowable federal safety limits. State and U.S. Food and Drug Administration inspectors are authorized to seize contaminated seafood, but officials conceded that the necessary time lag of at least two days in analyzing samples would mean that much of the product could hit retail markets and stores.

Earlier today, the governor and health officials said they did not believe a temporary return of Kepone infested fish posed an immediate health threat. But officials cautioned consumers against eating fish from the formerly off-limits waters.

"We're not saying that anyone who eats one or two fish will get cancer," said Robert Stroube, state director of health protection and environmental management. "But we are saying that no one can tell what the longterm health effects will be and we think a prudent person wouldn't eat them."

The fishing ban has been financially devasting to scores of fishermen and wholesalers in the James River and lower Chesapeake Bay area, who found thousands of the region's most fertile fishing areas declared off limits. Nearly a half dozen testified in court yesterday about the ban's economic impact and that they themselves had regularly eaten fish from the infested waters without health damage.

"The ban was a big joke," said Robert Brown, a Rescue, Va., fish wholesaler and one of the suit's plaintiffs, who said the ban had cost his business $10,000 a year in rockfish sales alone.

"We're not fools. I've been eating fish with this stuff (Kepone) for 15 years with no problem. I wouldn't sell anything that I won't eat," said Brown. He said he intended to buy fish from the formerly prohibited area and resell it to markets as far north as Baltimore and New York.

Health officials said the experience of the Kepone-eating fisherman are irrelevant because it could take 20 or more years for persons to develop cancer from the pesticide. "We don't know what will happen to them down the road," said Stroube.

Newport News Circuit Court Judge Henry D. Garnett ignored most of the health and economic arguments presented to him yesterday and overturned the ban on the narrow legal grounds that the state health board's emergency powers had been abused in continually reimposing the ban without public hearings and other administrative procedures.

Dalton press secretary Paul G. Edwards said the governor was reimposing the ban under the governor's own broad emergency powers to deal with immediate public health threats.

Del. Theodore V. Morrison of Newport News, attorney for the seafood interests challenging the ban, could not be reached for comment last night on the governor's move.