WASHINGTON (Reuters) - U.S. cities have become increasingly segregated by income since 1980, with the greatest divide in fast-growing Texas metropolitan areas, the Pew Research Center reported on Wednesday.

This rising segregation is seen in 27 of the 30 biggest metropolitan areas and stems from the long-term increase in U.S. income inequality, Pew said in its study of Census data.

Residential segregation by income is less widespread than segregation by race, even though U.S. black-white segregation has been falling for decades, the report said.

The analysis shows that 28 percent of lower-income households in 2010 were in a mostly lower-income neighborhood, up from 23 percent in 1980.

Eighteen percent of upper-income households were in a majority upper-income census tract, twice the level in 1980.

Rising income inequality "has led to a shrinkage in the share of neighborhoods across the United States that are predominantly middle class or mixed income," wrote Paul Taylor and Richard Fry, the report's authors.

The share of neighborhoods that are middle class fell to 76 percent in 2010 from 85 percent in 1980.

Among the 30 biggest metropolitan areas, San Antonio, Houston and Dallas show the greatest segregation of neighborhoods by income, the report said.

The least segregation was seen in Minneapolis-St. Paul; Portland, Oregon; and Orlando, Florida.

Most of the metropolitan areas showing the greatest increase in segregation have seen significant population growth as people moved in.

For example, Houston, Dallas and San Antonio are among the fastest-growing U.S. cities, with growth partly fueled by low-wage immigrants moving in along with better-paid high-skill workers and well-off retirees.

(Reporting by Ian Simpson; Editing by Will Dunham)