New York City Mayor Bill de Blasio (Andrew Burton/Getty Images)

New York Mayor Bill de Blasio’s push to provide universal preschool to the city’s 4-year-olds has so far disproportionately benefited children from middle- and upper-income families, according to a report released Wednesday that the mayor’s office is disputing.

In the first year of expansion, the number of pre-kindergarten seats in the city’s public schools increased by 36 percent in Zip codes where families earn more than the city’s average income of $51,865, according to the analysis of city data by researchers at the University of California at Berkeley. That was more than twice the rate of growth in the poorest quartile of city Zip codes, the report found.

The report questions whether the new slots are being allocated fairly and whether the imbalance could exacerbate the inequalities and achievement gap that the program seeks to address.

“The evidence is quite strong that pre-k lifts kids from poor communities. It’s much weaker in that it lifts kids from affluent or middle-class families,” said Bruce Fuller, a professor of education and public policy who directed the study and who has advocated for targeting preschool expansion efforts to poor children. “If we are going to move toward universal pre-k, let’s ramp it up carefully.”

De Blasio campaigned on a pledge to provide free, full-day pre-kindergarten for all of the city’s 4-year-olds. His push echoed national efforts to expand access to preschool to help children prepare for the increasing academic rigors of elementary school.

Wiley Norvell, City Hall spokesman, said that the Berkeley study is based on “errors and false assumptions.” Comparing the percent growth in various Zip Codes is ultimately misleading, he said, because many poor neighborhoods already had high numbers of prekindergarten seats.

“Our pre-K expansion is reaching thousands of low-income families in its first year and is providing this free, life-changing opportunity to families in greatest need,” he said.

The District funds universal preschool for all 3- and 4-year-olds. President Obama has been advocating for government-funded preschool programs for all children from low- and middle-income families, as education experts believe such programs can be tied to lasting social and emotional gains.

The New York mayor battled the state legislature to secure the necessary funds during his first nine months in office, and he then pushed for universal expansion within two years.

De Blasio’s administration added more than 23,000 slots, a combination of new full-day seats and the conversion of half-day seats to full day, by the start of this school year in public schools or community organizations, the report said. It was a significant boost from the 58,528 previous seats.

But higher-income neighbors and boroughs benefited the most from the first year’s roll out, the study found.

In the Bronx, the city’s poorest borough with a median household income of $32,568 in 2010, there was a 10 percent increase in the number of pre-k slots in school-based programs. By contrast, Queens, with a median income of $53,052, had a 36 percent increase in seats. Staten Island, with a median income of $70,560, had a 63 percent increase.

Community-based organizations host more than half of the new pre-k seats, but the study showed that the new seats were more concentrated in wealthier areas.

W. Steven Barnett, director of the National Institute for Early Education Research at Rutgers University who has seen the report, also questioned the geographically based analysis and said it could be misleading, since it does not reflect the actual number of low-income studnets that are served in each place.

The report suggests that the imbalance is tied to the greater availability of classroom space in more affluent neighborhoods and a stronger demand expressed by more economically secure families.

Fuller, who has studied preschool access across the country, said that middle-class mothers who work outside the home full time have been vocal advocates for preschool. “The demand is much softer in poorer communities,” he said, where mothers work part-time or irregular hours.