After five years of planning, public hearings and intensive lobbying, the Prince George's County Council tentatively voted yesterday to block the development of the proposed Mattawoman New Town, which would have brought 25,000 people into the rural southern end of the county.

The surprise decision came yesterday afternoon when the council, on a straw ballot, voted 7 to 4 to place more than 9,500 acres in Mattawoman. Brandywine and Cedarville areas into a rural agricultural zone - thus denying Washington Gas Light Co., the new town's developers, the zoning required to continue with their plans.

Aside from derailing the proposal for the controversial 2,500-acre new town, yesterday's tentative vote also apparently blocks the plans of several smaller developers who had planned to construct office buildings and small residential subdivisions in the surrounding area.

The decision, however, does not affect the construction of the proposed Mattawoman Mall, which is already under way. The mall was to be the centerpiece of the proposed new town to be developed by the Washington Gas Light Co.

The straw vote came as a surprise to both council members and observers, several of whom said they felt the Mattawoman New Town proposal had the support of at least six people on the 11-member council.

However, council member Samuel Bogley, who had been cosidered the swing vote on this issue, cast his vote against the proposal.

Bogley said after the meeting that he made his decision after a county transportation planner pointed out that the new development would mean that traffic on the already-over burdened roads system in the southern end of the county would reach critical proportions.

"I realize that everybody in the room was mildly shocked," Bogley said, "but when the traffic director of the Park and Planning Division told us the development would put 1,350 cars an hours on the roads down there, there was just no way I could consider approving it."

The development, which would be located just west of Rte. 5, below the intersection of Rte. 301 and Rte. 5 (also known as Branch Avenue), has received strong opposition from members of the surrounding community and from others who live along Branch Avenue north of the proposed development.

In a succession of the four-hour public hearings, possibly the lengthiest and most vocal in the council's history, area residents argued that the Mattawoman proposal would produce "a nightmare" of overcrowded roads, inadequate public facilities and environmental hazards.

Council member Floyd Wilson, who until yesterday's vote had been garded as a supporter of the new town, said he decided against the plan because of the public opposition.

"After the public hearings, it seemed an overwhelming mandate of the people to vote against it," Wilson said, "I feel the new town is a good idea for that area, but I'm not going to jam it down their throats."

William Beckett, the attorney for Washington Gas Light during the last five years of zoning negotiations, said he was "extremely disappointed" in the council's decision.

"I was surprised at the last-minute turnaround," Beckett said. "We have been working on this new town proposal for so long. We've put hundreds of thousands of dollars in the planning, and more than a million into land purchases."

The money poured into the WGL proposal does not affect the rates paid by WGL customers, Beckett said. "The project is a below-line item and any expenses or profits affect only the shareholders of the company," he said.

Beckett said his proposal still has a week. This is not the final vote. We are going to give some thought to what we can do to change their minds."

Several council members, however, indicated yesterday that , as long as the transportation planner did not change his figures as to the new town's impact on the county's roads, and council member, Bogley did not change his vote, the final vote on Tuesday would be identical to yesterday's straw vote.

Washington Gas Light Co. began buying large farms in the area in the early '60s, Beckett said. The company had planned to build a large underground gas storage facility underneath the property, he said, but when the plan proved unfeasible the company decided to develop the land into a residential and employment center.

The new town proposal - initially was linked with a J.C. Penney-Montgomery Ward proposal to build a regional shopping mall next to the WGL property. Last year the council approved a zoning request by Penney and Ward to build the mall and 300 to 500 town houses on 65 acres adjacent to the mall.

At the same time, it sent the Mattawoman New Town proposal back to the zoning board for further study.

The council's action does allow for industrial development on 1,200 acres on the east side of Rte. 301. County planner Art Tankersley said the council's decision would "establish an employment area and a commercial core before any massive residential development should begin."

Beckett said WGL Co. still has some options. "We can appeal the council's decision. We also have a pending zoning ease we could resurrect and we could come back here in six years when the council reviews the zoning in this area and try again."

Council member Gerard McDonough, who has supported the plan from its inception, said the council's action could be detrimental to the county's economic growth.

"If we approved this plan, it would give the southern part of the county an opportunity to contribute its share of the Prince George's economy. With all this low-density zoning, it will place more of a burden on developing the far northern and central part of the county. We have to get away from the sprawling subdivisions we have and into something new."