The holiday lull in the war between Fairfax County and surrounded but independent Fairfax City was broken yesterday by a salvo from City Hall.

City Attorney John H. Rust Jr. informed the city government that the county, despite its toughly worded intention of doing so, can't charge more for services to the city as long as contracts between the two jurisdictions are still in effect.

In an opinion given to City Manager George E. Hubler Jr., Rust wrote: ". . . the county has no authority to alter the remaining arrangements between the city and the county."

On Dec. 12, the county's top elected official, Board Chairman John F. Herrity, informed the city that if there is no agreement on new contract terms by March 1, the county will start charging the higher cost " . . . and should the city fail to pay promptly as billed, all of such services to the city will be terminated."

Herrity said that barring an agreement on school services, which mostly consists of county teachers working in city schools, would halt July 1, 1979.

Of other services - courts, prisoner detention, solid waste disposal, sewage treatment, libraries, public and mental health - Herrity said: " . . . the Board of Supervisors will take such action as it deems necessary to protect the interests of the county."

But Rust said none of the services, except for schools, can be canceled for at least three years. He said two services - sewage treatment and solid waste disposal - cannot be canceled at anytime.

Under the revised cost formula the county wants, the city would pay about $2 million more annually for services. According to the city, present charges total almost $10 million annually.

Hubler, in a memo to the City Council, estimated that the higher service charges would cost the average city taxpayer $169 more per year.

Rust said the city's response to the county ultimatum is up to the Council, which will meet Jan. 3. But he said, "I would suspect the city would go to court and get an injunction" to block the county from charging more for the services it provides.

The city already has decided to take over the fire and rescue service the county has been providing. The takeover will occur in May.

The city has estimated that an independent school system, while it would be more expensive than the present arrangement with the county, would be cheaper than the formula the county says it will start using March 1.