The income gap between the wealthiest U.S. households and the middle class widened from 2010 to 2011, according to a U.S. Census Bureau report.

Annual income increased 1.6 percent for the richest 20 percent of Americans, the report found. And within that percentile, the top 5 percent gained 4.9 percent. Meanwhile, income declined for middle class Americans, but held steady for the poorest.

The average household income in 2011, after inflation, was $50,054, marking a 1.5 percent decline from the 2010 average. That figure is 8.9 percent lower than a 1999 peak and 8.1 percent lower than the average income in 2007, the year before the recession swept the country.

The Census Bureau's report also found that the nation’s official poverty rate in 2011 was unchanged from its 2010 rate, holding steady at 15.0 percent, with an estimated 46.2 million people living in poverty. This comes after three years of increases in the number of poor Americans.

A household of two working-age adults and two children was considered poor in 2011 if its yearly income dropped below $22,811. But some researchers noted that this threshold doesn't take into account other factors like cost of living differences across different regions.

"The poverty rate is useful for comparing trends over time, but it doesn't do a good job of setting a bar for how much money families need to get by," Jennifer Romich, director of the West Coast Poverty Center, said in a statement. "In spite of its imperfections, what the poverty measure does tell us is that over 46 million people are living with incomes below a very low threshold."

Follow LiveScience on Twitter @livescience. We're also on Facebook & Google+.