Former welfare recipients and homeless people are missing from the federal food stamp program even though they qualify for aid, according to two reports released this week.

The first report found that two-thirds of the families who left food stamp rolls in 1997 remained eligible for the program, with former welfare families much more likely to leave the rolls than families that had not been receiving cash assistance.

"Whether the families leaving welfare assumed that they no longer qualified for food stamps, or whether they chose to leave the program when they began working, is unknown," said the report by the Urban Institute's Assessing the New Federalism Project. The study was based on a 1997 national survey of 44,000 families.

The second report charged that many homeless people are illegally being denied food stamps because they are without permanent addresses.

Of 72 charities that responded to the survey released Tuesday by the National Law Center on Homelessness and Poverty, 45 percent said their homeless clients were sometimes or frequently denied food stamps for lack of addresses. The surveys were given to homeless shelters and other organizations that had reported problems with the program.

Federal officials said it is illegal to deny food stamps based on lack of an address and promised to look into the situation.

Officials have long known of the problems with former welfare recipients.

Last summer, the General Accounting Office, Congress's investigative arm, reported that some state and local governments were making it too difficult for families leaving welfare to retain food stamps. It also found that the number of children who received food stamps fell faster than the number of those living in poverty, "indicating a growing gap between need and assistance."

The Clinton administration launched a public education campaign last summer designed to build enrollment in the program, aimed primarily at the working poor.

The Urban Institute report documents the problem.

People leave the food stamp rolls for a variety of reasons, including getting jobs that pay enough so they are no longer eligible or needy. The report found, however, that people who had been on cash welfare were much more likely to leave food stamps than those who had never been.

That problem was particularly acute for the poorest Americans, those earning less than half the poverty rate. Of them, 45 percent who had been on welfare left food stamps, compared with 23 percent of those who had never been on welfare.

"What is absolutely clear," the report concludes, "is that families that left welfare joined a population of working poor families that have always had low rates of food stamp participation, even though food stamps offer significant help in paying for food."