Federal, state and local income taxes will consume an ever-increasing share of the average family's income between now and the turn of the century, according to a Library of Congress study.

Over the same period the government sector of the economy - which accounted for 30.8 per cent of total gross national product in 1975 - will grow to 37.5 per cent of total output by the year 2000, according to projections by the Congressional Research Service.

The study was prepared at the request of Rep. Charles Vanik (D-Ohio), a member of the tax-writing House Ways and Means Committee.

In a statement, Vanik said the study does not reflect the "substantial and growing burden of social security and medicare taxes." He warned that the study shows "that we must be extremely careful in starting new programs and amending existing ones."

He said Congress must "debate more clearly the question of how large a role government should play in our lives in the future.'

According to the study, the federal income tax will "remain the largest tax paid by the median income family" and will "increase considerably as a per cent of the family's income."

In 1975, median family income was $13,719 and federal income taxes were 10.9 per cent of that income, or $1,496.

By 1985, median family income will have risen to $30,588.60 - more because of rising prices than because of rising standards of living - and federal income taxes will climb to 13.9 per cent of income or $4,254.34.

The federal government will take an even bigger bite of family incomes by the end of the century, when it will claim $11.351.20 of the then median income of $77,687.40.

That is 14.6 per cent of family income.

While nominal incomes will rise more than 5 1/2 times in the quarter century between 1975 and 2000, inflation will eat up most of the increase. In 1975 dollars, factoring out the impact of inflation, median family incomes will rise from $13,719 to $25.690.30, according to projections in the study.

The congressional study is based on an econometric model developed by the economic research firm. Data Resources Inc., based in Cambridge, Mass. It is only a projection and cannot fully account for changes in tax laws, government policies or major economic changes.

Nevertheless, such projections can provide reasonably accurate indications of where the economy and federal and state policies are heading.

Not only does the federal income tax bite rise from 10.9 per cent to 14.6 per cent of median family income, state and local income taxes rise from 1.9 per cent in 1975 to 4.4 per cent of income.

Sales and property taxes remain almost constant. In 1975, these taxes accounted for 5.3 per cent of total median income. At the turn of the century they will take up 5.4 per cent.