For the second time in less than nine months, the District of Columbia's largest automobile insurer--Government Employees Insurance Co.--has raised the cost of policies, this time by an average of 6.9 percent.

Geico filed the new rates Friday with the D.C. Insurance Department and put them into effect Monday. The average cost of a policy covering one car for comprehensive, collision and property damage liability will rise by $24 a year, from $346 to $370, according to O.M. (Tony) Nicely, Geico's regional vice president.

The higher rates will be applied as individual policies are renewed. Nicely said Geico issues close to one-quarter of all policies in the city, more than twice the number of its closest rival.

Under current D.C. law, higher insurance rates proposed by a company go into effect automatically on a specified future date unless blocked by the department, and they may later be rolled back if they are found to be excessive or unduly discriminatory.

Acting D.C. insurance superintendent James R. Montgomery III said Geico's swift imposition of higher rates over a weekend "to say the least, is not the usual procedure."

An official notice inviting public comments on the increase will be published in the D.C. Register, probably Sept. 24, Montgomery said.

Geico's most recent increase, an average of 4.2 percent effective last Dec. 21, was protested by one policyholder, but later was upheld by Montgomery.

Nicely said higher rates are needed because people are driving more, resulting in more accidents, and because there has been a 16.7 percent rise in automobile theft claims in the past nine months.