Mexico - The flow of immigrants into the United States from Mexico has come to a standstill and may have reversed, according to a study released on Monday.

The report by the non-partisan Pew Hispanic Center found that an influx that brought 12 million immigrants to the United States since the 1970s, more than half of whom came illegally, began to slow five years ago and may have reversed in the past two years.

"The standstill appears to be the result of many factors, including the weakened US job and housing construction markets, heightened border enforcement, and a rise in deportations," the report states.

Other factors include the growing dangers associated with crossing north illegally from Mexico and the long-term decline in Mexico's birth rates. The report said it was possible the immigration wave would resume as the US economy recovers.

An estimated 11.2 million illegal immigrants, more than half of them from Mexico, live and work in the shadows in the United States. The issue of what to do with them has become a hot-button issue in the United States in recent years.

President Barack Obama promised to deliver comprehensive immigration reform, tightening security on the border while offering millions of illegal immigrants who learn English and pay a fine, the chance to become US citizens. So far he has failed to deliver.

Republican White House hopeful Mitt Romney, meanwhile, supports tough enforcement. He has expressed support for Arizona's state law that is cracking down on illegal immigrants. That law is subject to an appeals hearing in the US Supreme Court on Wednesday to determine whether the state strayed too far into the federal government's powers.

Former President George W. Bush brought the last attempt at an immigration overhaul to a vote in 2007, but it was killed off by Republicans in the US Senate.

The Pew study, which drew on both US and Mexican government data, found that the sharp downward trend in net migration from Mexico began about five years ago and has led to the first significant decrease in at least two decades in the population of illegal immigrants from Mexico.

As of 2011, some 6.1 million Mexican immigrants were living illegally in the United States, down from a peak of nearly 7 million in 2007, according to Pew estimates based on data from the US Census Bureau.

Over the same period, the population of legal immigrants from Mexico rose modestly, from 5.6 million in 2007 to 5.8 million in 2011.

"While it is not possible to say so with certainty, the trend lines within this latest 5-year period suggest that return flow to Mexico probably exceeded the in-flow from Mexico during the last year or two," the study found.

Around 29 percent of all current US immigrants are Mexican-born. The next largest sending country — India — accounts for just 4.5 percent of the estimated 40 million immigrants currently in the US.

Beyond its size, the most distinctive feature of the modern Mexican wave has been the unprecedented share of immigrants who have come to the United States illegally. Just over half, or 51 percent, of all current Mexican immigrants are in the country without legal permission, while some 58 percent of the estimated 11.2 million unauthorized immigrants in the United States are Mexican, the study found.