Almost seven in every 10 foreign prisoners in US jails are Mexicans, a US government report has revealed.
A report by the US Government Accountability Office also states that between 2005 and 2010, there was a 7% increase in the number of foreign inmates in federal prisons.
Local prisons, have seen a 35% increase in the number of jailed foreigners.
The GAO reports that 68% of the 55,000 foreigners in US prisons are Mexican nationals.
In total, foreigners make up more than 25% of the US prison population.
Some fear that the new report will put pressure on the government to adopt stricter anti-immigration laws.
Most of the prisoners were convicted for immigration offences (65%), followed by drug-related crimes and drug trafficking.
The GAO estimates that foreign prisoners cost US taxpayers up to $1.6bn (£972m) per year.
The cost per capita varies according to each state. In Texas, the government spends $12,000 per inmate, but in California this figure triples.
In California and other border states, specialists warn that the number of illegal immigrants in jail could jeopardize the local prison system.
Jails managed by state governments can benefit from a plan that entitles local governments to recover a part of the costs of keeping illegal immigrants in jail.
Created in 1994, the State Criminal Alien Assistance Program (SCAAP) pays back a part of the spending when some inmates meet certain requirements, such as being detained for a minimum number of days.
But these requirements are not always met, which has lead to calls for reform.
Tim Donnelly, a Republican in the California State Assembly, says California spends $885 million each year on "undocumented prisoners".
He argues that "the federal government is not doing its job of deporting these criminals to their countries of origin".
He also suggests an "exchange program" with the neighbouring government to send Mexican inmates to Mexico in exchange for the smaller numbers of American prisoners held there.
"This would release much needed space in the prisons of our country," says Donnelly, a well-known activist against illegal immigration.
Some NGOs who campaign for the rights of immigrants have warned that the number of illegal migrants arrested has risen sharply - 85% more in 2010 than in 2005.
"Some are detained simply because of [a] traffic misdemeanour," Jennifer Allen, of the NGO Borderaction, told the BBC.
"In some states, such as Arizona, a traffic misdemeanour may result in deportation."
According to estimates by the Department of Homeland Security, out of the 25.3m foreigners living in the United States in 2009, around 10.8m are considered to be in the country illegally.