The number of households forced to spend  more than half of their monthly income on rent and possibly also live in severely substandard conditions rose by more than 20 percent between 2007 and 2009, the federal government said Tuesday.

Almost 7.1 million low-income households were defined as having “worst case housing needs” as a result of joblessness, rising rents and a general lack of affordable rental housing.

The dramatic increase was the largest in any previous two-year period since at least 1985, when the  Department of Housing and Urban Development began collecting data on renters whose incomes are less than 50 percent of the area median income and who are not receiving government rental assistance.

Hispanics and families with children were the most affected in 2009, the HUD report found. Forty-five percent of all very low income Hispanics faced worst-case needs, as did 39 percent of families with children, including another 550,000 that weren’t considered to be in the most dire situations in 2007.

Unemployment and underemployment pushed more than 410,000 households into a worst-case housing situation in 2009.

“The housing crisis and the subsequent foreclosure crisis have put strain on the rental market,” said Raphael Bostic, HUD’s  assistant secretary for policy development and research. “Many homeowners who were not in the (rental) market have been thrown into this market, increasing the competition for units. Very low income renters paid 10.6 percent in 2009 than in 2007.”

Bostic noted the study did not include the 4.4 million households that receive financial rental assistance.