A D.C. City Council committee has approved a measure that would strengthen the city's laws against drunken driving and modify the penalty structure for persons convicted of driving while intoxicated.

The bill defines a blood alcohol level of .10--a level achieved by consumption of about five ounces of liquor in an hour's time--as absolute proof of drunkenness. Present law defines that level as representing only a presumption of drunkenness that must be proved in court.

The bill, approved Wednesday by the committee on transportation and environmental affairs, would change the range of penalties that could be imposed for drunken driving.

The penalty for a first offense would be reduced from a maximum of six months in jail and a $500 fine to a maximum of three months and a $300 fine.

A council staff member said the lesser penalty was seen as a possible inducement for prosecutors, who sometimes are reluctant to seek a harsh penalty for first offenders, to prosecute more drunken driving cases rather than allow first-offender suspects to plead guilty to a lesser offense.

The bill, introduced by Polly Shackleton (D-Ward 3) and cosponsored by Jerry A. Moore (R-At Large), would increase the maximum possible penalty for a second offense to a year in jail and a $5,000 fine. Present law allows a maximum sentence of a year's imprisonment amd a $1,000 fine. A third conviction under the proposed bill could lead to imposition of a term of a year in jail and a $10,000 fine.

The measure is expected to come before the full Council late this month or early in June. Seventeen persons died in alcohol-related accidents in the District in 1981, and more than 3,600 persons were arrested on alcohol-related charges.