The administration wants to have the hydrogen cars on the market and available to consumers at an affordable price by 2020.

However, the report by a panel at the National Academy of Sciences shows that Americans should not hold their breath waiting for the cars to arrive in showrooms.

"In the best-case scenario, the transition to a hydrogen economy would take many decades, and any reductions in oil imports and carbon dioxide emissions are likely to be minor during the next 25 years," said the academy, an independent group that makes scientific recommendations to Congress.

The emissions-free vehicles would cut pollution and emit water as their only by-product. Automobiles currently emit large quantities of carbon dioxide, a heat-trapping gas that scientists have linked to global warming.

The Bush administration's 2005 budget request asked Congress for $228 million to develop cars that run on hydrogen fuel and the service stations to support them, up 43 percent from the 2004 request.

An Energy Department spokesman was not available to comment on the report.

Environmental groups said the government needs to take quicker action to reduce gasoline use by boosting mileage requirements and curb growing emissions from new gas-guzzling SUVs, pickup trucks and minivans.

"We simply can't bank on hydrogen alone to cut our dependence on Middle East oil or fix the global warming problem," said Antonia Herzog at the Natural Resources Defense Council, pointing out that Americans will buy 450 million new cars and trucks before the hydrogen car is available.