The state's top analyst on Tuesday not only questioned the legality of launching a high-speed train, but also warned legislators that starting construction on the rail line could be a $6 billion waste of tax funds at the expense of social services, education and other transportation projects.

In the sharpest critique yet of the state's newly revised plan to spend two decades and $99 billion building a bullet train line, the Legislative Analyst's Office bashed planners for relying on "highly speculative" funding sources.

As a result, the analyst concluded that it's "highly uncertain" the full project will ever get built. Even so, the state intends to start construction in the Central Valley next year by spending $2.7 billion in state bonds plus a $3.3 billion federal grant to build a stretch of track too short for bullet train service -- a move that has already triggered a lawsuit. But passengers won't start zipping between San Francisco and Anaheim unless Congress bankrolls more than half the project, a dubious scenario considering federal lawmakers have killed all high-speed rail funds for two straight years.

"It appears doubtful that substantial additional federal support will be forthcoming anytime soon," the report says. "This makes it increasingly likely that the (initial stretch of track) may be all that is ever built," a project that is "unlikely to justify (the) expense."

The report, unveiled at an Assembly oversight hearing, appears to give lawmakers the strongest ammunition yet to kill the project instead of starting construction, which would bury the state even deeper in debt.

"You may also have to look at making other cuts to social services programs or education," and not funding other transportation projects, report co-author Farra Bracht told the Assembly Committee on Transportation.

Even the officials backing the project conceded the review has merit.

"High-speed rail certainly faces a challenge that it does not have a dedicated revenue source like the gas tax," said Dan Richard, one of Gov. Jerry Brown's appointees to the California High-Speed Rail Authority board. "If the federal government chooses not to continue to fund high-speed rail, it's going to be very difficult to see how we can complete this."

The analyst's nine-page report also concluded that the rail authority's estimate of the cost to scrap the rail line, and instead to expand freeways and airports, was "overstated" and not realistic. Further, they say it's "unproven" that high-speed rail would really solve the state's future transportation demand, that their economic impact study is "incomplete and imbalanced" and that rail authority staffing is "inadequate."

The analyst further warned that the rail line voters approved in 2008 only allows construction to begin when officials outline committed funding and environmental clearances for a segment long enough to run service. But the rail authority has neither, the analyst said. Kings County, where construction would start, just sued to stop the project on similar grounds, though the rail authority maintained Tuesday that its plan is legal.

With Brown supporting the plan -- his appointees helped draft it -- and most legislative Republicans opposing, the decision will come down to the will of Democratic lawmakers. While they made no decisions or major pronouncements at the hearing, some officials continued to express reservations.

"My fear is that with (an uncertain) price tag and no dedicated revenue stream, any money we do get will go to that project, to the detriment of the state's existing transportation systems," said Assemblywoman Bonnie Lowenthal, D-Long Beach, the committee chairwoman. Yet, she said high-speed rail promises "extraordinary benefits such as jobs, private investment and economic growth that has to be considered if we're to look at this honestly."

Assemblywoman Joan Buchanan, D-San Ramon, a committee member, said voters might wonder why the money isn't being spent on infrastructure that needs work now.

"In the meantime, my constituents (are) stuck on a freeway that's like a parking lot, when we could be using the money to extend BART," Buchanan said.

Still, Assemblyman Jose Solorio, an Anaheim-area Democrat, told his colleagues that they couldn't let this opportunity slip from their grasp.

In a brief statement released after the hearing, the rail authority welcomed the critiques and vowed to meet with the analyst's office "to discuss the issues for legislative consideration raised in the report."

Contact Mike Rosenberg at mrosenberg@mercurynews.com.