Prince George's County Council yesterday reaffirmed last week's tentative decision to reject Washington Gas Light Co.'s proposal to build a new town that would have brought 35,000 people to the rural southern portion of the county.

By a unanimous vote, the 11-member council approved a comprehensive reasoning of the land in the Brandy wine, Mattawoman and Cedarville areas, including provisions that would place the bulk of the WGL property in an agricultural zone, thus precluding the development of the proposed Mattawoman new town.

WGL, had proposed building the new town on 2,500 acres southeast of the intersection of Rtes. 5 and 301. According to their proposal, by the mid-1980s the development would have included 6,000 homes, an office, research and development park and a number of retail stores.

Under the zoning approved yesterday, the land can be used only for farming or for residential development two-acre lots.

Until last week's tentative vote, it appliedto most council observers that WGL would receive the zoning it needed to build the new town.

But then two council members who were regarded as supporters of the new town proposal, Samuel Bogley and Floyd Wilson, unexpectedly voted against the plan. Their decision came after a traffic report by a county planned indicated the road network in that region would be unable to cope with the new growth.

When it became obvious that the proposed Mattawoman project would go down to defeat yesterday, other new town supporters on the council reversed their votes and made the decision unanimous.

The council's action also will make available for industrial and commercial development more than 1,500 acres of land for industrial and commercial development south of the Rte. 301/Rte. 5 interchange, including the proposed Mattawoman Mall, which was to have been the core of the new town proposal.