In an effort to stop what it claims is deceptive advertising, the Federal Trade Commission yesterday sought a federal court order prohibiting Brown & Williamson Tobacco Corp. from claiming that its Barclay cigarettes contain only one milligram of tar.

A commission source said the FTC is disputing the advertisements on grounds that Brown & Williamson have not used accepted testing methods to determine its low tar rating, and that consumers would be misled by its claims.

By a 1971 agreement, cigarette manufacturers agreed to disclose the tar and nicotine levels of their products using FTC reports or by using FTC test methods.

Brown & Williamson, however, recently switched to a non-FTC study as the basis for its claims of low tar, the commission source said. Consumers, not used to distinguishing between test methods, may be confused into thinking Barclay's low-tar claims are just as valid as other claims, he added.

The FTC move is an effort not only to prevent what it sees as deceptive advertising, the source said, but also to prod Brown & Williamson into complying fully with an FTC ruling reached last summer.

The ruling said because of the unique design of Barclay cigarettes, the FTC testing procedures could not certify Barclay's tar levels.

Before the FTC could enforce its findings, though, Brown & Williamson appealed to U.S. District Court in Louisville and succeeded in blocking enforcement.

Last April, though, the Sixth Circuit U.S. Court of Appeal lifted the stay on the FTC, and the commission formally asked Brown & Williamson to stop using FTC test results to substantiate its tar claims, the source said.

However, Brown & Williamson continued to use advertisements claiming one mg. tar for Barclay cigarettes and using FTC substantiation, the source said. When they recently switched to other substantiation, the commission filed the complaint, he said.