The method used by Fairfax County's school system to teach English to 2,300 foreign-born students violates federal guidelines and could lead to a cutoff of federal aid, school officials were informed yesterday.

The county and federal officials are embroiled in a dispute over methods and goals for improving the students ability to speak and write in English.

Fairfax favors classes conducted in English. The U.S. Department of Health, Education and Welfare's Office of Civil Rights, which yesterday put Fairfax officials on notice, favors a bilingual approach.

In a letter to Fairfax superintendent S. John Davis, HEW yesterday gave county officials 20 days to furnish evidence in support of its method. The matter otherwise will be referred to HEW's administrative enforcement office, the agency said. That would be the first step in a lengthy process that could lead to the eventual cutoff of HEW money to the county schools.

Davis yesterday characterized the demand as "ludicrous . . . We don't intend to drop everything we're doing to respond to this letter."

Fairfax County schools received more than $17 million from HEV last year, most of it federal impact funds to alleviate the burden of a heavy population of federal employes in the area.

Davis called "outragous" the fact that HEW can ask the school system to establish a bilingual program. A bilingual approach has never been proved a more effective method, he said.

"I don't understand why the burden of proof should be on our shoulders," Davis added. "Philosophically we disagree. To ask us to maintain the youngsters' ability in his native language is appalling."

There are 73 nationalities represented among the 2,200 Fairfax students for whom English is a second language, according to a county school official, and 26 languages in which 20 or more students are fluent.

According to Davis, a bilingual program like that favored by HEW would cost $3 to $5 million a year to operate, compared to the $1 million it costs the county to run its current operation.

Foreign-born county students now are taught English in a program called "English" as a Second Language" (ESL) in classes conducted strictly in English.

In the bilingual program recommended by the federal government, students would be taught not only English but other classroom subjects using the student's native tongue.

A bilingual approach "delays progress in learning to speak English," Davis said yesterday. "I am firmly convinced that ESL is the most appropriate."

Davis said he does not intend to change the country's methods.

Fairfax County operates its own limited pilot program in which 160 of its foreign-born students are given some subjects, like math and science, in their native language.

Dorilz Katz, of HEW's regional civil rights office in Philadelphia, said yesterday the agency hoped the country would expand that special program.

Davis cited increased costs, however, and the possibility of cross-county busing as reasons for not doing so.

There are 54 teachers in the ESL program and bout 20 teacher aides in the pilot program, a school official said. Of the 54 teachers in ESL, 44 are bilingual or trilingual and represent 18 languages.