Trump intends to renegotiate NAFTA as early as August, letter to Congress says The letter means negotiations can begin as soon as 90 days, or August 16.

 -- The White House has formally told Congress that the Trump administration plans to renegotiate the North American Free Trade Agreement, starting a countdown clock until talks between the United States, Canada and Mexico can begin in August.

"NAFTA was negotiated 25 years ago, and while our economy and businesses have changed considerably over that period, NAFTA has not. Many chapters are outdated and do not reflect modern standards," reads a letter from U.S. Trade Representative Robert Lighthizer to Congressional leadership on Thursday. "I am pleased to notify the Congress that the President intends to initiate negotiations with Canada and Mexico regarding modernization of the North American Free Trade Agreement (NAFTA)."

The letter begins a 90-day window before beginning formal negotiations as early as August 16.

"The United States seeks to support higher-paying jobs in the United States and to grow the U.S. economy by improving U.S. opportunities under NAFTA," the letter reads.

Trump made criticism of NAFTA a staple of his presidential campaign last year, asserting that the United States was at a disadvantage in trade with its northern and southern neighbors. Trump blamed lopsided trade relationships around the globe for the loss of manufacturing jobs and lackluster economic growth.

The agreement was signed under President George H.W. Bush in 1992 and implemented under President Bill Clinton in 1993. Trump has threatened to withdraw from the pact if negotiations are unsuccessful.

Mexican Foreign Secretary Luis Videgaray Caso said that Mexico welcomes the negotiations, that his nation was informed by the White House of the process every step of the way and that his government will approach the process constructively.

"We understand that this is a 25-year-old agreement," he said at a State Department summit on drug cartels with Secretary of State Rex Tillerson on Thursday. "The world has changed."

House speaker Paul Ryan also acknowledged the letter and his intention to work with the Trump team.

"I welcome the administration’s effort to improve and update NAFTA for the 21st century economy," Ryan said in a statement. "Congress looks forward to working hand-in-hand with the Trump administration to achieve the best deal possible for American workers and our economy."

Senate Minority Leader Chuck Schumer said the move was a "welcome first step, but the devil will be in the details."

"So far, this administration’s trade policy has been characterized by a lot of talk and no action," he said in a statement. "I hope this will change."

Canada or Mexico rank as the first or second largest export market for 30 of the 50 states in the country, according to information on the U.S. Trade Representative's website. The page says American manufacturing exports have climbed more than 250 percent, and exports of computers, furniture, paper and fabricated metals have tripled since 1993.

ABC News' Conor Finnegan contributed to this report.