The Fairfax County Board of Supervisors, faced with an adverse court ruling, yesterday reversed a decision they made in June and ordered County Executive Leonard Whorton to release a controversial consultant's study of proposed sewer rate increases.

The report reveals that unless Fairfax County gets U.S. funds to build a -21 million sewage pipeline the length of the county, sewer service rates will have to be increased 56 per cent.

Currently the service charge is 95 cents per 1,000 gallons of sewage. That rate would have to be increased to $1.51 for every 1,000 gallons by Nov. 1 if the county is to pay for the pipeline, according to the study.

Release of the report was, in part, a victory for Supervisor Audrey Moore (D-Annandale) who had sought the report's release since last spring. Mrs. Moore had threatened to lend her name to the court action if the report wasn't released.

Yesterday's vote by the Board came after last week's decision by Fairfax Circuit Court Judge Barnard F. Jennings ordering the county to release the report within 30 days. The suit to release the report had been filed by environmental activist Marian Agnew, a frequent ally of Mrs. Moore.

The study, done by Alexander Potter Assoc., says that if the diversion pipeline gets the uncertain federal funding, the service rate would have to be increased only 16 per cent - from the present 95 cents per 1,000 gallons to $1.11 effective, Nov. 1. This smaller increase would cover the escalating costs of advanced sewage treatment.

Whorton, in amemorandum accompanying the summary of the Potter report, said that the report was not released last April because it wa "incomplete and still is so adjudged." In court, the county argued that the report did not have to be released under the Virginia Freedom of Information Act, as demanded by Mrs. Agnew, because it was a "working paper" and had not yet been sent to the Board of Supervisors.

In ordering the study released, the supervisors took no position on sewer rate increase. Instead they asked the county staff to continue its present study and return some recommendations.

Last spring Whorton told the board that the rapidly increasing cost of treatment to meet federal clean water standards was wiping out the surplus in the sewer fund, and would produce a $1.6 million deficit this fiscal year.

Despite Whorton's early warning, movement on a rate increase has beem stalemated by uncertainty over the funding of the pipeline project, called the Difficult Run pumpdown.

The project resulted from a court decision ordering Fairfax to provide sewage treatment for growth in the northwestern corner of the county, primarily the Herndon-Reston area.

Last year the prospects for federal funding of the pumpdown seemed bright, but not approval is in doubt because the project may not be able to meet all requirements of the Clean Water Act.

Mrs. Moore said that she would press next week for the smaller rate increase. "The longer that's put off," she said, "the bigger the increase is going to be . . . It's irresponsible to have gone as long as we have."

Several others supervisors indicated that they would not support raising the sewer service charge to cover the $21 million pumpdown. The only alternative, if federal funds don't come, would be increasing the sewer hookup charge for new home. At present, Fairfax has one of the lowest hookup charges in northern Virginia.