An attorney for four civic groups seeking to block construction of I-66 from the Beltway through Arlington to the Potomac River said this week that the groups will decide soon whether to appeal a 4th U.S. Circuit Court of Appeals decision that would permit the construction.

"We think the decision is legally weak in several respects," said Hank Levine, a member of the law firm that is representing the civic organizations.

A three-judge panel of the appeals court last week unpheld a lower court decision permitting construction of the controversial portion of the interstate highway. Work already has begun on that part of the four-lane limited access road which is to run from the capital Beltway in Fairfax County to near the Theodore Roosevelt Bridge across the Potomac.

Two options open to opponents of the highway are an appeal for a rehearing by the full court of appeals instead of the usual three-judge panel, or to ask the Supreme Court to hear the case, Levine said.

In upholding the decision of the federal district court in Alexandria, the appeals court said, "The secretary of transportation has complied with all of the applicable environmental protection statutes in approving the final I-66 proposal."

Opponents of the I-66 proposal had charged that Brock Adams, the Carter administration's transportation secretary, and William Coleman, who held the position in the Ford administration, exceeded their powers in approving the plans for the four-lane, 9.7 miles extension of the road.

The judges noted that the appeals court had blocked construction of I-66 in 1972 because the secretary of transportation had failed to comply with environmental laws. The court said the proposals for the highway have been modified since then.

One of the opponents' major arguments was that the environmental impact statement Coleman considered before making his decision on the highway did not consider that the Metro rail line, designed to go down the middle of I-66, might never be built beyond Glebe Road in Arlington.

The appeals court said the Federal Highway Administration "properly concluded" that abandonment of the rail line was not sufficiently likely to merit detailed discussion in the enviromental impact statement. The extension of Metro along I-66 is still under study.

The opponents also argued that the environmental impact statement failed to discuss sufficiently four options they considered reasonable substitutes for I-66, that the environmental impact statement was based on arbitrary and biased projections of car pool formations, and that Coleman did not comply with the National Historic Preservation Act.

The court found no merit in those arguments.