Superior Court Judge Michael Kenny said the California High-Speed Rail Authority "abused its discretion by approving a funding plan that did not comply with the requirements of the law."
Yet he declined to immediately halt funding for the project and said he will hold another hearing to determine what happens next.
The California High-Speed Rail Authority and the office of Gov. Jerry Brown, who has championed the project, did not respond immediately to requests for comment.
Proposition 1A required the agency to identify funding for the entire first segment of the project and clear all environmental hurdles before starting construction.
Lawmakers last summer authorized selling $2.6 billion in state bonds for construction of the first 130-mile segment in the
The judge said the authority's 2012 draft business plan describes all potential federal sources of financing as "theoretical possibilities and not as sources of funds reasonably expected actually to be available starting in 2015."
The judge said the plain language in the initiative indicates that financing and environmental clearances should be completed for the first 290 miles.