The White House formally notified Congress on Monday of its intention to bring Mexico into negotiations on a free-trade agreement with eight other countries in the Asia-Pacific region.
"Mexico's participation ... further increases the economic significance of a TPP (Trans-Pacific Partnership) agreement," U.S. Trade Representative Ron Kirk said in a letter to congressional leaders.
A second letter informing Congress of plans to bring Canada into the talks is expected in coming days.
Both Mexico and Canada were invited into the negotiations last month by the nine current TPP countries: the United States, Australia, New Zealand, Chile, Peru, Singapore, Malaysia, Vietnam and Brunei.
It is longstanding U.S. practice for the White House to formally notify Congress before entering into actual trade negotiations with any country.
Mexico is the United States' third-largest trading partner, behind Canada and China. Two-way goods trade last year totaled $460.6 billion, or more than double U.S. trade with its fourth-largest trading partner, Japan.
Mexico and Canada are already free trade partners with the United States under the North American Free Trade Agreement, which went into force in 1994.
In his letter, Kirk touted Mexico's entry into the TPP as opportunity to update that 18-year-old pact with a "cutting-edge agreement ... that will further enhance our trade relationship and benefit U.S. workers, manufacturers, service supplies, farmers, ranchers, small businesses and consumers."
Current TPP members are holding their 13th round of negotiations this week in San Diego, with an eye toward reaching a final agreement by late 2012 or early 2013. (Reporting by Doug Palmer; Editing by)