The number of illegal immigrants in the United States has leveled off but may be on the rise again, following a sharp drop that accompanied the start of the Great Recession, according to a report released Monday.

The analysis from the Pew Research Center estimated that 11.7 million immigrants were living in the country illegally in March 2012. That was down from an all-time high of 12.2 million in 2007 — a year before the stock market collapsed — but it represented a slight increase from an estimated 11.3 million in 2009, the worst year of the recession.

Analysts cautioned that the figures are inexact and represent the midpoint of ranges based on U.S. census surveys and other data.

“The 2012 population estimate . . . in a statistical sense is no different from the 2009 estimate,” authors Jeffrey S. Passel, D’Vera Cohn and Ana Gonzalez-Barrera wrote. However, they said, there is little doubt that the decline of illegal immigrants during the recession has “bottomed out.”

The report could lend new urgency to the debate in Congress about comprehensive immigration reform, as lawmakers wrestle with how to address the fate of the undocumented population.

The Democratic-controlled Senate has approved a far-reaching plan to add new layers of security along the U.S.-Mexican border, provide more legal channels for immigration and allow those here illegally to gain citizenship over 13 years if they pay fines, learn English, remain employed and do not commit crimes.

But the Republican-led House has shelved that plan and is focused instead on a series of piecemeal bills that do not include measures to legalize undocumented immigrants. The Pew report does not make a recommendation on a path forward for legislation.

Immigration advocates said the report lends credence to their argument that giving unauthorized immigrants legal status, and potentially citizenship, would boost the economy by adding millions of new taxpayers.

“It’s clear that migration is fluctuating with our economy,” said Ali Noorani, executive director of the National Immigration Forum. “We could really use 11.7 million new taxpayers. The only thing standing between here and there is Congress passing reform.”

But those who oppose offering citizenship to unauthorized immigrants said the report showed that enforcement measures along the border and in the workplace are not sufficient and should be the sole focus of new legislation.

“We may not have the same magnitude of illegal flow as we did in the 1990s and mid-2000s, when we reached a crescendo, but that does not mean the problem has gone away,” said Mark Krikorian, executive director of the Center for Immigration Studies. “The idea that the current level of enforcement is somehow adequate is not true.”

The number of people living in the United States illegally has skyrocketed since 1990, when there were an estimated 3.5 million unauthorized immigrants. An overhaul of immigration laws in 1986 — which granted about 2.5 million people a path to citizenship — didn’t stem the flow of immigrants who came to the United States illegally to look for jobs or to join family members here.

A Pew study last year showed that the net flow of immigrants across the U.S-Mexican border had fallen to nearly zero. The Obama administration, building on programs implemented under President George W. Bush, ramped up enforcement measures that led to record numbers of deportations. Last year, the Immigration and Customs Enforcement agency deported nearly 410,000 people.

The Obama administration has hailed such figures as evidence that enforcement programs are working, but Senate negotiators included billions of dollars for new border control measures in their immigration bill to woo Republicans. The legislation passed in June on a bipartisan vote of 68 to 32.

Opponents of immigration reform have argued that the decrease in that population has been driven not by better enforcement but by economic factors, as the U.S. economy hurtled into a recession with fewer jobs.

The Pew study released Monday found that the number of Mexican immigrants living illegally in the United States has dropped steadily to just over 6 million, down from a high of 6.9 million in 2007 and the lowest number since 2004. By contrast, the number of unauthorized immigrants from other nations has reached an all-time high of 5.7 million, the study found.