WASHINGTON — The number of people living in neighborhoods of extreme poverty grew by a third over the past decade, according to a new report, erasing most of the gains from the 1990s when concentrated poverty declined.

More than 10 percent of America’s poor now live in such neighborhoods, up from 9.1 percent in the beginning of the decade, an addition of more than two million people, according to the report by the Brookings Institution, an independent research group.

Extreme poverty — defined as areas where at least 40 percent of the population lives below the federal poverty line, which in 2010 was $22,300 for a family of four — is still below its 1990 level, when 14 percent of poor people lived in such areas.

The report analyzed Census Bureau income data from 2000 to 2009, the most recent year for which there was comprehensive data.