Prince William County dropped its long opposition to any expansion of the Manassas National Battlefield Park and agreed yesterday to a compromise plan that would add 620 acres to the Civil War park.

The 4-to-3 vote by the board of supervisors seems likely to end a dispute so bitter it had been dubbed "The Third Battle of Manassas" and to allow for passage of Congressional legislation that will block commercial development from some areas near the battlefield.

County officials for four years had refused to support any measure calling for expansion of the 3,000-acre park, site of two famous Civil War battles.

With yesterday's vote, it appeared that key obstacles to expansion, sought by preservationists and history buffs, have been removed.

Sen. Harry F. Byrd Jr. (I-Va.) said yesterday that the compromise developed by Sen. John W. Warner (R-Va.) "seems to be a reasonable one, and obviously a majority of the Prince William board agrees." An aide said Byrd most undoubtedly will become a co-sponsor" of the compromise legislation.

Rep. Herbert E. Harris II (D-Va.) has introduced bills in each of the last three sessions of Congress to expand the park by up to 1,715 acres. Each time the House has passed the measure, only to have it blocked in the senate by either Byrd of Warner's predecessor, Sen. William L. Scott (R-Va.).

The supervisor's support for the Warner compromise was initiated by Supervisor Donald L. White, a long-time opponent of expansion, who insisted that his motion include a stipulation that Warner's expansion is "the most acceptable to this board."

White, vice chairman of the board, said he is not "any real advocate of expanding the park," but proposed the motion because "it is quite clear there will be a bill on the Senate side, and if we are going to get a bill, the Warner proposal has the least negative impact on Prince William County."

The vote in favor was 4 to 3, with Chairman Kathleen K. Seefeldt, Alice E. Humphies and J. J. McCoart joining the majority. Opposing expansion were Supervisors James Byrd, A. J. Donnelly and T. Clay Wood.

Seefeldt, who favors the more sweeping Harris plan, said Warner's proposal "provides the basis for a good compromise" if separate bills are passed by the House and Senate.

Warner had outlined his compromise plan at a meeting with four of the supervisors in Washington last week. In a letter to the entire board, Warner wrote: "I believe this an approach that not only protects historically valuable land, but also takes into account the needs of Prince William County.

The major difference between Warner's plan, and the Harris legislation is that Warner excludes from acquistion by the National Park Service commercially desirable property paralleling Interstate Rte. 66 on either side of State Road 234.

Warner said "compatible development at the Intersection of Rte. 234 and I-66, and the construction of a Rte. 234 bypass, eliminates much of the traffic on the present road through the park, are worthy projects."

Both Warner and Harris would add to the park about two-thirds of the Brawner farm, which most historians agree is the most historical parcel not now within the bounds of the park. The two proposals also call for purchasing the Stone Bridge battle site at the eastern end of the park.

Omitted from Warner's bill are about 450 acres in Fairfax County, on the north and east sides of Bull Run. Harris wants the Department of Interior's National Park Service to place scenic easements on that land.

Warner said reducing the size of the expansion also will cut the costs "at least in half," a factor that is "of paramount concern" in getting Congressional approval. An aide to Warner estimated that the cost of the proposal would be about $5 million, as compared to $19.7 million for the Harris bill.

While Harris lauded Warner for "taking the significant step" of sponsoring some form of park expansion, the congressman, whose district includes the park, said he has no intention of dropping his own proposal.

An aide said yesterday that Harris feels "very strongly that the entrance to the park (along I-66) should be protected from industrial use."