Of 8.8 million women in the United States with children under 21 whose fathers were not living at home, only 2.1 million of the mothers received full child-support payments from the absent father, the Census Bureau reported yesterday.

The rest of the mothers, according to the survey of 1985 data, lacked a child-support order, or, despite having one, received no money or less than the stipulated amount.

Payments for those women receiving child support averaged $2,220 annually -- down from $2,530 in 1983 after adjustments for inflation.

The report is certain to reinforce the growing consensus among policymakers that nonsupport by absent fathers is an important cause of poverty, requiring strong new laws.

Wayne A. Stanton, administrator of the the Department of Health and Human Services' Aid to Families With Dependent Children program and director of the Office of Child Support Enforcement, said the numbers show that "full use of child-support enforcement laws is crucial to ensuring that millions of children receive the support they need and deserve . . . . We cannot do enough to help these vulnerable children."

Sen. Daniel Patrick Moynihan (D-N.Y.), sponsor of welfare legislation that includes new enforcement provisions, said, "These figures show the need for automatic wage withholding. The failure of absent parents -- fathers 90 percent of the time -- to support their own children is a leading cause of poverty in America."

Although the proportion of Americans with income below the government's official poverty line was 13.6 percent in 1986, the bureau report found that it was 32 percent in the 8.8 million families with absent fathers.

Moynihan's bill requires automatic deduction by employers each payday of child-support payments owed by their employes, even if the individual is not in arrears on payments. It includes provisions to establish paternity as a step toward obtaining child-support orders. Some of the same provisions are included in major House welfare bills sponsored by Republicans and Democrats.

An aide to House Minority Leader Robert H. Michel (R-Ill.), sponsor of a GOP welfare bill, said, "This report indicates what we have felt all along -- a great deal of the welfare problems are related to mothers and children having been abandoned by the fathers . . . . We've got provisions to beef up the laws . . . . "

The Census Bureau report said that of the 8.8 million mothers, 3.4 million had no child-support order at all, although about half wanted them.

Another 1 million had support orders or agreements but they did not call for payments in 1985 for a variety of reasons, such as death of the absent father or children exceeding the age of eligibity.

That left 4.4 million with valid court orders or agreements entitling them to child-support payments in 1985, the bureau said, but only 2.1 million were actually receiving the full amount due.

Another 1.1 million were getting partial payments, and the remaining 1.1 million were getting nothing at all despite having a valid order.

The bureau found that the 4.4 million women who were entitled to receive payments in 1985 were due a total of $10.9 billion from the absent fathers, but the actual amount received totaled $7.2 billion.

"Significantly greater child-support enforcement efforts are needed immediately to ensure that all children receive the maximum support to which they are entitled," said Mary Lee Allen of the Children's Defense Fund.

Rep. Nancy L. Johnson (R-Conn.), a cosponsor of the Michel bill, said through an aide, "The failure to collect child support is one of the major reasons so many female-headed families are on welfare. The Michel bill includes new provisions to help states establish paternity and includes immediate mandatory wage withholding."

The census survey showed huge differences among race and class. For example, four-fifths of the women who were divorced or remarried had child-support awards, and most of them received something. But less than one-fifth of never-married single mothers had such awards, and the amount actually received was only half the $2,220 average of all women receiving payments.

Seven of 10 white mothers had been awarded support, but only one-third the black mothers and two-fifths the Hispanic mothers.

The report also looked at alimony payments, finding that of 19.1 million women in 1985 who had ever been divorced or were currently separated from their husbands, only 840,000 were due alimony payments in 1985 and 616,000 were receiving them. The average payment was $3,733 a year.

Among these women, however, 5.9 million had received property settlements -- three-quarters in the form of houses, real estate, cars and furnishings.