COLUMBUS, Ohio (AP) _ Bus service in the city resumed its first regular schedule today since drivers and mechanics were ordered to end a three-day strike, but the union said it may walk out again next month.

Glenna Watson, the Central Ohio Transit Authority's assistant general manager for operations, said all 90 drivers expected to work Thursday showed up for the abbreviated holiday schedule. The regular weekday schedule then resumed today.

About 290 buses were to be on the streets today busing 90,000 people, officials had said earlier.

Late Tuesday, the State Employment Relations Board ruled that the union for the 630 drivers and mechanics failed to file proper notice of the strike with the transit authority and therefore the walkout was illegal.

Transport Workers Local 208 officials filed notice Wednesday saying the union intends to strike again Dec. 8 if differences over wages and other issues are not worked out by then.

Negotiations will resume next week, said federal mediator Joseph Santa- Emma.

The strike began at 12:01 a.m. Monday after the union voted 552-12 to reject transit authority's contract offer. Both sides have said they will not waver on contract demands.

COTA has offered a 5.4 percent increase over 2 1/2 years, while the union seeks a 15 percent increase over three years. The two sides also disagree on sick leave and other issues.