Poverty has deepened in Boston’s poorest neighborhoods, widening the gap between the city’s wealthiest and neediest residents, a report being released today finds.

The study points to concentrated need in Dorchester, Mattapan, and Roxbury, where 42 percent of children live in poverty, the densest cluster of childhood poverty in the state, according to the study sponsored by the Boston Foundation.

In those communities, 85 percent of families are headed by a single parent, mainly mothers, and at least 20 percent of the adults have no high school diploma.

Poverty there is fueled by unemployment and low educational attainment, the study found.

“We no longer have conditions for the American dream; that actually does not exist anymore,’’ said Tiziana Dearing, executive director of the antipoverty organization Boston Rising, who had not read the report. “It is, in fact, harder now than it used to be.’’

In 1990, 24 percent of African-American children across the city lived in poverty. Nearly two decades later, that figure has risen to 35 percent, the study indicated.

The report, which found that poverty citywide remained constant in the past two decades, confirms recent studies by the US Census Bureau and the Brookings Institute finding that despite substantial progress against concentrated poverty during the 1990s, much of those gains have been erased in the turbulent 2000s.

The report ties Boston to a global trend linking race, class, and income disparities. It also illustrates the toll exacted by the depressed economy in extremely poor communities plagued by inadequate health care, lack of jobs, and persistent crime.

National policies have not been kind to families on the lower rungs of the economic ladder or to industrial workers, their jobs automated or sent offshore, officials at the foundation said.