Updated at 3:01 p.m. ET
New tariffs against Mexico will begin to bite next week, President Trump vowed Tuesday, unless the White House is satisfied that Mexico's government is acting with new alacrity to stop migrants from crossing into the United States.
"This will take effect next week, 5%," Trump said during his visit to London.
Trump said he is open to continuing negotiations with Mexican leaders, including at a meeting scheduled for Wednesday between its foreign minister and Secretary of State Mike Pompeo.
However, the president is also willing, he said, to impose the tariffs during the ongoing negotiations to make it clear that he is serious.
"I think Mexico will step up and do what they should have done," Trump said at a news conference with British Prime Minister Theresa May after a meeting with her at 10 Downing Street.
Trump said he won't accept any explanations about the difficulty for Mexican authorities of constraining the flow of migrants from Central and South America.
"I don't want to hear that Mexico is run by cartels and drug lords and coyotes," he said. "I don't want to hear about that. ... Mexico should step up and stop this onslaught, this invasion into our country."
Trump was asked about the prospect that Republicans in Congress might try to block tariffs.
"I don't think they will do that," he said. "I think if they do that, it would be foolish."
Even so, later on Tuesday in Washington, Senate Majority Leader Mitch McConnell, R-Ky., said he hoped Mexican tariffs could be avoided. Most of the weekly meeting of Senate Republicans was a discussion with White House representatives about GOP concerns about the president's plans, McConnell said.
"There is not much support in my conference for tariffs," McConnell told reporters.
When pressed again about any potential effort to block the president's action, the majority leader emphasized about Senate Republicans: "We're not fans of tariffs," and he repeated that he hoped that talks between the U.S. and Mexico would find a way to avoid them.
Senate Minority Leader Chuck Schumer, D-N.Y., predicted the president would back off of the plan after hearing about the opposition from the business community and lawmakers.
Trump at Downing Street
Trump and May concluded a series of meetings with general commitments to pursue a new trade deal — although apparently without one in place.
A free trade agreement has taken on new urgency for May and Britons as they eye the United Kingdom's pending exit from the European Union, which has jangled the British economy and its politics.
Brexit difficulties also are why May is nearing the end of her premiership and a leadership contest is underway within the Conservative Party to replace her.
Trump hasn't been shy before and during his visit about expressing his thoughts about a successor. He spoke by phone with former Foreign Secretary Boris Johnson and said on Tuesday that he thought Johnson "would do a good job."
May, for her part, stressed that the U.S.-British relationship would remain important no matter who becomes the next prime minister.
The leader of the Labor Party, Jeremy Corbyn, wanted to meet with Trump, but the president declined, Trump said. It wasn't immediately clear whether that was correct.
Corbyn has faulted Trump for his political positions and his criticism of British political figures. He also declined to attend the state dinner for Trump at Buckingham Palace given by Queen Elizabeth II on Monday evening.
On Tuesday, Corbyn addressed anti-Trump protesters in Central London around the time of the news conference between the U.S. and British leaders.
Trump also continued his criticism of London Mayor Sadiq Khan, in what has amounted to a running war of words between the two men over the course of Trump's trip.
Khan responded to Trump's tweets on Tuesday morning by telling CNN that the American president was acting like an "11-year-old" but that he didn't feel "it's for me to respond in a like manner."
Trump was asked about Khan and his responses to the president's earlier comments in his news conference with May.
The president said Khan "has not been a not very good mayor from what I understand" and blamed him for an increase in crime in the metropolis.
Commitment to the special relationship
Although Trump and May did not appear to conclude the "substantial" trade deal that the president said he supports, he did soften the line his administration has taken with London over a thorny security dispute: China's mobile technology giant Huawei.
Washington has pressured its allies around the world to bar Huawei from developing new 5G mobile networks because of security concerns about Beijing's ability to exploit them for its own purposes.
Huawei denies that its telecom equipment can be exploited in this way or that it's an arm of the Chinese government.
The British government, sensitive about its relationship with Beijing, has been reluctant to go along with American restrictions. Pompeo threatened last month that Britain's ability to share intelligence information with the United States might be on the line.
But Trump said on Tuesday that he wasn't contemplating any cutoff for Britain because he's sure the two sides will come to an agreement on Huawei.
"We're going to both have absolutely an agreement on Huawei and everything else," he said. "We will be able to work out any differences. I see absolutely no limitations. We've never had any limitation."