WASHINGTON -- President Barack Obama's administration on Monday formally invited Mexico to join controversial free trade negotiations known as the Trans-Pacific Partnership, reinforcing consumer and environmental group concerns that the deal may establish broad new powers for corporations.

The invitation comes less than one week after a leaked U.S. document sparked an uproar because it proposed terms that would grant corporations the ability to bypass U.S. public interest laws and regulations in courts here and appeal directly to international tribunals. U.S. advocacy groups and some members of Congress have criticized the U.S. proposals.

The U.S. has granted corporations similar powers under trade pacts extending back to the 1993 North American Free Trade Agreement with Mexico and Canada. But the language in last week's leaked negotiation text broadened key definitions that would allow more companies to challenge a wider spectrum of rules. The text also would make rules surrounding government contracting -- a $1 trillion market in the U.S. -- eligible for corporate challenges before international tribunals.

Although Mexico does not have trade agreements with several of the countries involved in the Trans-Pacific talks, it does have deals with Peru, Chile and the U.S., all of which are involved. NAFTA already allows for trade without tariffs between the U.S. and Mexico, creating concern among public interest groups that the Trans-Pacific deal will establish broader deregulation.

"Because Mexico already has zero-tariff trade agreements with various TPP countries, including the U.S., adding Mexico just highlights that this is not really about trade, but about deregulation and new corporate rights and it provides no basis for U.S. officials to claim expanded U.S. exports," said Lori Wallach, director of Public Citizen's Global Trade Watch. Public Citizen is a consumer advocacy group.

"The invitation for Mexico to join the TPP talks shows just how expansive this trade agreement could be, making it even more critical that the agreement includes strong and enforceable environmental protections and prevents corporations from attacking environmental and public interest policies in private tribunals," Ilana Solomon, trade representative for the environmental group Sierra Club, told HuffPost.

Last week, the Office of the U.S. Trade Representative issued a statement declaring that, "Nothing in our TPP investment proposal could impair our government's ability to pursue legitimate, non-discriminatory public interest regulation, including measures to protect public health, public safety and the environment."

Nevertheless, consumer groups have noted that, if ratified, the terms of the leaked text could establish a very high cost for those regulations: new trade sanctions directed by an international tribunal. Under the proposal, companies would be able to appeal directly to an international body, a status once reserved for sovereign nations.

Members of Congress have taken the Obama administration to task for the secrecy surrounding the Trans-Pacific negotiations, after their offices were denied access to draft documents. More than 600 corporate officials and a handful of nonprofit groups have seen the texts through memberships on government trade advisory boards, and those advisers are not permitted to share information on the documents with the public. Most public interest organizations have only been made aware of the deal's terms through leaks of negotiation proposals.

Public health experts have been especially vocal about their dissatisfaction with the Trans-Pacific proposals, noting that intellectual property terms of the deal would increase the price of lifesaving medications.

Republican presidential candidate Mitt Romney has spoken favorably of NAFTA and other U.S. trade pacts, and has urged Obama to sign the Trans-Pacific deal quickly.