Counting prison inmates anywhere but in the prisons where they are incarcerated would cost the Census Bureau about $250 million and would threaten the accuracy of the 10-year population count, a new report to Congress says.

The Census Bureau has always counted inmates as local residents in the communities where prisons are located. The system adds to those communities' populations, giving them additional clout when it comes to divvying up government grant money and laying out legislative districts.

Some big-city congressmen, whose districts typically lose in the deal, want the inmates counted as residents of their home towns and cities in the 2010 Census.

Many rural areas, where most prisons are located, would lose big chunks of population. Some cities would gain a lot of new "residents."

Congress told the Census Bureau to study the issue, but the response was not encouraging.

"Our study raises concerns that this change would result in decreased accuracy for a possibly large proportion of millions of individuals confined on Census day," the bureau's report to Congress says.

Rep. Jose Serrano, a Democrat from the Bronx in New York City, blasted the census report on Thursday, saying the bureau "abdicated" its responsibility to provide accurate demographic data of the country.

"Instead of providing an overview of how they could make this change in methodology work, the Census Bureau only makes excuses for why they don't believe it should be done," Serrano said in a statement. "Our nation deserves the very best demographic data. With this report, the Census abdicated responsibility for providing that data."

The issue is getting bigger because the country's prison population — in state, federal and local facilities — has ballooned to more than 2 million people.

Serrano argues that inmates should be counted in the communities where they will likely return after their release.

"These are the communities that must help released prisoners reintegrate into society, and these are the communities that are most in need of gaining an accurate picture of their populations, in order to help them assess their funding priorities," Serrano said.

But officials in communities that house prisons said they deserve any benefits they can get for housing criminals from other areas. Those communities have to live with the stigma of housing prisons, as well as the fear of escapes.

Census takers could use administrative records or interviews with inmates to determine their home addresses, the census report said. But costs and security concerns would make interviews difficult, and administrative records often are incomplete, the report says.

"Often, there is a street number and street name missing, and only the city and state are available," the report said. Census takers need exact addresses to assign residents to legislative districts.

The bureau estimates it would cost $250 million to interview inmates and to process their information.