The official statement was expected Friday, according to the Washington Post, which noted that "some White House aides are trying to talk him out of it" by citing further danger to international financial markets and even the USMCA trade agreement proposed between the two nations and Canada.
However, Trump instead decided to announce the trade penalty in a Thursday evening tweet.
On June 10th, the United States will impose a 5% Tariff on all goods coming into our Country from Mexico, until such time as illegal migrants coming through Mexico, and into our Country, STOP. The Tariff will gradually increase until the Illegal Immigration problem is remedied,..— Donald J. Trump (@realDonaldTrump) May 30, 2019
"On June 10th, the United States will impose a 5% tariff on all goods coming into our country from Mexico, until such time as illegal migrants coming through Mexico, and into our country, STOP. The tariff will gradually increase until the illegal immigration problem is remedied, at which time the tariffs will be removed. Details from the White House to follow," Trump wrote.
In a subsequent document released by the White House, Trump noted that the tariffs would double to 10% on July 1, and then another 5% at the beginning of every subsequent month after that until they reach 25%.
"Tariffs will permanently remain at the 25 percent level unless and until Mexico substantially stops the illegal inflow of aliens coming through its territory. Workers who come to our country through the legal admissions process, including those working on farms, ranches, and in other businesses, will be allowed easy passage," the statement reads.
In comments to reporters outside the White House earlier Thursday, Trump described an anticipated statement "having to do with the border and having to do with people illegally coming over the border and it will be my biggest statement so far on the border" as "major" and "big league."
"We are going to do something very dramatic on the border, because people are coming into our country," he said.
Shortly after the Washington Post story broke, US Vice President Mike Pence said the president was considering additional action to make it clear Mexico must do more to stop the flow of migrants.
It's not clear if Trump has the authority to impose such tariffs under the terms of the USMCA deal, Trump's rehash of the 1994 North American Free Trade Agreement (NAFTA), which he blasted during his campaign as unfair to Americans.
Mexico, Canada and the US reached a tentative deal on the USMCA last September. A Statement of Administrative Action is expected this week, which would set in motion the process of Congressional approval of the treaty. Canadian Prime Minister Justin Trudeau's Liberal Party introduced legislation in the Canadian parliament Wednesday to ratify the treaty, and Mexican President Andres Manuel Lopez Obrador presented the documents to the Mexican Senate on Thursday.
Trump has made both the trade deal and immigration from Mexico and other Central American countries the keystone of his domestic policy, calling immigration a national emergency.
"The asylum procedures are ridiculous," Trump said Thursday. "No place in the world has what we have in terms of ridiculous immigration laws."
Over 100,000 migrants have been detained at the border in the past two months, according to the Department of Homeland Security. A group of 1,036 Central American migrants were detained Wednesday near El Paso, Texas — the largest such group US border agents have ever arrested.