Warning of an "immense financial risk" to the state, a renowned group of transportation and financial experts advised lawmakers Tuesday to pull the emergency brake on funding California's $99 billion high-speed train.

The Legislature created the high-speed rail peer review group to weigh the project's chances for success, and its sobering conclusions are the most striking -- and perhaps most influential -- analysis yet of the pivotal plan lawmakers will evaluate in coming months.

Former Caltrans chief Will Kempton, the group's chairman, said most troubling was the state's plan to spend all available funding -- $6 billion -- on a small stretch of track in the Central Valley and hope for the rest of the money later.

"There is simply just no identification of a long-term funding source or commitment, and we think that is a fundamental flaw," Kempton said. "There is a huge risk they could be stuck with (only) that initial section of track."

The report's release coincides with Wednesday's start of the Legislative session, which will end with a vote this June on whether to authorize $2.7 billion in state bond funds to match $3.3 billion in federal money.

"It is particularly compelling because this comes from a group with no political ax to grind, and undeniable experience and expertise on a global basis," state Sen. Joe Simitian, D-Palo Alto, who has helped lead the discussion on the project in Sacramento. "It adds to an accumulating pile of concern."

Though Gov. Jerry Brown continues to support the project, opposition among the public and the Legislature is mounting, particularly as the cost to build has tripled since voters approved the railroad in 2008.

As state leaders face a federal deadline this fall to start construction, the report is likely to fuel criticism.

"It's a voice we're certainly going to need to listen to," said Assemblyman Rich Gordon, D-Menlo Park, who heads the budget committee overseeing the project.

State Sen. Mark DeSaulnier, an East Bay Democrat and head of the Senate transportation committee, agreed with the eight-page report, siding with Simitian, most Republican lawmakers and a growing number of Democrats who argue the current plan shouldn't be funded.

But in a 10-page response, the California High-Speed Rail Authority attacked the credibility of the peer review panel and called the analysis "by and large deeply flawed."

They accuse the group of holding the bullet train to an impossible funding standard, noting other megaprojects have been built segment-by-segment without having all the money in place at the onset.

"As the report presents a narrow, inaccurate and superficial assessment of the high-speed rail program, it does a disservice to policymakers who must confront these decisions," project Chairman Tom Umberg said.

But Kempton noted that other huge projects, like the Interstate system, had dedicated sources of future funding, such as a gas tax, that the high-speed rail plan does not.

The report is particularly surprising because panel members support high-speed rail in concept. The railroad is envisioned to zip passengers between San Francisco and Anaheim by 2034.

"It was very hard for me," said Kempton, currently the head of Orange County's transportation agency. "I do believe that we should have a bold vision for transportation but I think that vision has to be tempered with reality, as well."

Their assessment also calls into question whether the plan to start building in the remote Central Valley is legal, since the tracks would be too short to support bullet train service. They also express doubt over project cost estimates, whether the rail authority is equipped to handle the project and if the train would actually turn a profit.

The rail authority denied virtually all the accusations, saying the group's assertions were "unsupported," didn't mention the potential benefits of the project and would let massive matching federal funds slip away.

Staff writers Steve Harmon and Denis Cuff contributed to this report. Contact Mike Rosenberg at 408-920-5705.