Design work on a light-rail Purple Line and a dedicated express bus route in the Interstate 270 corridor would stop after June 30 if the Maryland General Assembly does not approve a state transportation tax increase, according to state budget plans.

The Maryland Department of Transportation’s recently released six-year capital budget would reallocate $41.6 million in final design money for the proposed Purple Line and $17.2 million in preliminary engineering funds for the proposed Corridor Cities Transitway if no state transportation-related tax increase passes. The 16-mile Purple Line would run between Bethesda and New Carrollton. The 15-mile Corridor Cities Transitway would provide an express bus link between the Shady Grove Metro station and upper Montgomery.

Word that both projects might be put on hold after this fiscal year caught local transit advocates by surprise and prompted the Montgomery County Council to protest the plan in a letter Wednesday to the acting Maryland transportation secretary, Darrell B. Mobley.

If state funding ends, the council wrote, “these two lines will stop dead in their tracks. After more than a decade of work on these two projects, this is unacceptable.”

Leif Dormsjo, acting Maryland deputy transportation secretary, said the state still supports both projects, as well as a proposed light-rail Red Line in Baltimore, but will not allot money to work that it cannot afford to complete.

“It’s a reflection of reality — there’s no way to sugarcoat it,” Dormsjo said in an interview. “Without a revenue increase, it doesn’t make sense to proceed into a phase of design that we can’t complete.”

As part of a lengthy and rigorous process to win highly competitive federal aid, the state must show how it plans to pay for construction of all three transit projects. The Purple Line is estimated to cost $2.15 billion. The Corridor Cities Transitway is estimated to cost $545 million. The Red Line is projected to cost $2.57 billion. State officials have said that they need federal money to cover half of the construction costs.

Money for the Purple Line’s final design would cover buying right of way, relocating utility lines, and preparing detailed cost estimates and bid documents. With funding for preliminary engineering, planners on the Corridor Cities Transitway would help refine the design and cost estimates and complete the federally required environmental review.

“It seems like the state is saying, ‘If we don’t get the money in 2013, we’re done with you,’ ” said Montgomery Council member Roger Berliner (D- Potomac-Bethesda), chairman of the transportation committee. “The state has done nothing to ensure we get the money.”

Berliner said he faults Maryland Gov. Martin O’Malley (D) for not introducing legislation this year to restore the state’s depleted transportation fund.

O’Malley’s proposal last year to apply the state’s 6 percent sales tax to gas failed. Senate President Thomas V. Mike Miller Jr. (D-Calvert) has proposed a 3 percent sales tax on gasoline and a way for local officials to add 5 cents to the state’s 23.5-cents-per-gallon gas tax to raise money for road and transit projects.