The gap between income groups widened from 1980 to 1983, as America's wealthiest families boosted their share of national after-tax income at the expense of other households, according to a report released yesterday by House Budget Committee Chairman William H. Gray III (D-Pa.) and Joint Economic Committee Chairman David R. Obey (D-Wis.).

The report, based on a Census Bureau study of after-tax income of households in 1983, said the one-fifth of American households making up the top income bracket -- those with after-tax income above $29,763 in 1983 -- received 42 percent of all after-tax income in 1983, compared with 40.6 percent in 1980.

The report said all other groups of households had a lower share of national income after taxes in 1983 than in 1980.

The lowest fifth of households received 4.7 percent of after-tax income in 1983 compared with 4.9 percent in 1980, the report said.

The next-to-lowest fifth fell from 11.6 percent to 11.1 percent.

The middle fifth fell from 17.9 percent to 17.3 percent.

The fourth group fell from 25.1 percent of all after-tax income to 24.8 percent.

The report said the increase for the top fifth represented a shift of $25 billion in income -- about $1,480 per household more than if the income distribution had not changed since 1980.

For those at the top -- the 5 percent with household income above $46,288 after taxes -- the shift meant $3,320 more income per household than if the distribution had not changed.

The report said 1980 tax changes that reduced income taxes for the well-to-do were responsible for about half the increase in the share of national after-tax income received by the highest-earning families, while economic conditions were responsible for the rest.

One result of the shift in distribution, the report said, was that although the average income per household, in constant dollars, increased by 5.4 percent from 1980 to 1983 for the the top fifth of households -- from $39,890 per household to $42,043 -- it also dropped 1.5 percent to 2.5 percent for the lowest three-fifths.

The average income rose 0.7 percent for the fifth of households just below the top-earning group, the report said.