The troubled Colonial Transit commuter bus line, which daily carries more than 200 Prince William County residents to Northern Virginia Metro stations, is declaring bankruptcy and will end regular service today, company president Hal James said.

But commuters will continue to be served, according to an announcement from the recently formed and competing Washington Motor Coach Co., which has been carrying another 200 Prince William commuters directly into Washington.

Late yesterday that company announced it is acquiring more buses this weekend and plans to pick up all Colonial Transit riders on Monday at the regular stops and times.

Prince William County had received a $1.4 million state grant to buy 20 new buses and lease them to a private operator, but Colonial Transit's James said, "That was going too slowly to help us . . . . We were losing $200,000 a year."

Some of Colonial Transit's 26 employes are attempting to organize a new bus company, "but it's unlikely" anything would come of that before next week, if it succeeds at all, James said.

In its heyday in 1978, Colonial was carrying more than 3,500 commuters a day, on routes from Reston as well as Prince William County.

But Reston commuters transferred to a special Metrobus commuter run and ridership on the Prince William runs declined precipitously because of van pooling and car pooling, which have received state and local government support.

Colonial was down to fewer than 1,000 daily riders earlier this year and lost many of them when it joined Metro in the practice of stopping buses at Northern Virginia subway stations and ending runs directly into Washington.

Colonial also dropped its fares, from $23 to $17 a week, for the round-trip 25-mile commute, but hundreds of riders abandoned Colonial for the new Washington Motor Coach Co.

Colonial still has 56 buses, but only five were in operating condition yesterday, James said.

That meant four of the company's nine regularly scheduled runs had to be canceled--not an uncommon occurrence since all the buses are 20 to 30 years old.

The company plans to honor the 13 charter bus runs it has scheduled for Saturday, probably subleasing the charters to other companies if it can't get enough buses running by then, James said. Charter bus operations were what had been keeping Colonial Transit alive until this year, James said earlier this summer.

Charles Nixon, president of Washington Motor Coach, said yesterday his company has grown to nine runs daily--the same as Colonial Transit--and that he is acquiring new bus drivers and three more buses this weekend, bringing his total to 16.