The agreement, negotiated last year between the US, Canada and Mexico, violated American environmental laws, District Judge Charles Richey said.
He barred President Bill Clinton from presenting the pact to Congress for approval.
The ruling, which apparently took the White House by surprise, could delay final approval of the treaty, signed last December by George Bush, the former president, and the leaders of Canada and Mexico, for several months and perhaps even years.
Judge Richey said the treaty as it now stood 'will have significant environmental effects and may worsen the environmental problems already existing in the US-Mexico border area'.
A full environmental impact report would have to be prepared by the administration before it could go forward with the agreement, he concluded.
The development adds to the controversy surrounding the treaty, which President Clinton wants fully approved by the end of the year. Environmental issues and fear of job losses to Mexico have stirred considerable grass-roots opposition to the deal in the US.
While it seems likely that the administration will appeal against the decision, a spokesman for the President would not comment yesterday, saying he had not had enough time to consider the implications of the ruling.
The case was brought to court by a coalition of environmental lobby groups that have been trying to publicise the possible consequences of the treaty going into effect.
As now drafted it would by-pass some US environmental standards such as pesticide residue limits and pollution controls.
While President Clinton is committed to implementing the treaty, his administration has engaged in supplementary negotiations with the two other countries.
These talks have centred on finalising chapters on labour movement limitations and environmental controls. The talks so far have proved heavy going.Reuse content