A key regional planning agency recommended yesterday a controversial expansion in the hours for car-pool restrictions on Rte. I-66 inside the Capital Beltway in an attempt to reduce traffic congestion.

The Transportation Planning Board, a coordinating panel affiliated with the Metropolitan Washington Council of Governments, called for extending the morning and afternoon rush-hour periods during which the highway is limited to cars carrying at least three persons.

Under the board's proposal, the car-pool requirement would be imposed on eastbound traffic from 6:30 a.m. to 9 a.m. and on westbound traffic from 3:30 p.m. to 6:30 p.m. At present the car-pool restriction is in effect from 7 a.m. to 9 a.m. and from 4 p.m. to 6 p.m.

A spokesman for Rep. Frank R. Wolf (R-Va.), who won congressional approval of a 1983 measure that shortened the car-pool periods, said Wolf will oppose the panel's recommendation. Wolf believes the car-pool rules should be limited to two hours in the morning and two in the afternoon, the spokesman said.

Under the congressional measure, the rules also were relaxed to permit use of the road by car pools carrying three persons rather than four. The planning board recommended retaining the three-person requirement until congestion on Rte. I-66 becomes severe, probably in the early 1990s. Then, it said, the four-person requirement should be reinstated.

The board's recommendations are expected to play a key role in an extensive review of the issue by federal and state officials. Under a 1977 Transportation Department ruling, any major shifts in I-66 inside the Beltway must take into account the views of the planning board and Metro officials.

The Metro transit system has not yet commented on the issue. Nevertheless, some Metro officials have expressed concern that changes in car-pool rules on I-66 may have an impact on the number of subway riders who use the new Orange Line stations in the Rte. I-66 median, which are scheduled to open next summer.

If car-pool rules are relaxed, more commuters might drive instead of taking Metro trains, officials said. If the rules are tightened or if traffic congestion increases, more commuters might switch to the rail system.

According to federal officials, Secretary of Transportation Elizabeth Hanford Dole may reach a decision on the issue as early as November. Any move is expected to be closely coordinated with Virginia highway officials.

The planning board's recommendations were patterned largely on proposals by a consulting firm, JHK & Associates, which presented a report on the issue this year.

The study said an expansion in car-pool hours is warranted, partly because of increased congestion before and after the rush-hour periods.

Nevertheless, the planning board departed from the consultants' recommendations in urging that afternoon restrictions begin at 3:30 p.m. Under the JHK plan, the rules would take effect at 4 p.m.

The planning board argued that congestion now has a "detrimental impact" on trips on Rte. I-66 and to Dulles International Airport between 3:30 p.m. and 4 p.m.

In another development, the planning board relased a report recommending a new shuttle bus service along Rockville Pike in Montgomery County between Metro's Grosvenor and Rockville rail stations.

The report, by Urban Mobility Corp., a consulting firm, said the service should be financed partly by private businesses.