President Trump threatened on Thursday to impose a 35 percent tariff on Canadian imports, upending negotiations between the countries that had Canada’s representatives hopeful that a trade deal could be reached in a matter of weeks.

Mr. Trump posted a letter to Prime Minister Mark Carney of Canada on social media outlining his latest threat. He said the new, higher tariff rate for Canada would go into effect on Aug. 1, though it was unclear if it would affect all Canadian goods, or if he would follow through. Mr. Carney’s office did not immediately respond to requests for comment about Mr. Trump’s letter.

The letter to Canada resembled the format of letters Mr. Trump has sent to nearly two dozen American trading partners this week. But it deviated from the other letters by accusing Canada of having “financially retaliated” against the United States with its own tariffs. It also raised Mr. Trump’s repeated assertion — unsupported by American and Canadian data — that Canada had not done enough to stop the flow of fentanyl across the U.S.-Canada border, as well as his complaints about Canada’s high tariffs on U.S. dairy farmers.

Mr. Trump also suggested that the new tariffs were subject to further negotiations ahead of the Aug. 1 deadline. “These Tariffs may be modified, upward or downward, depending on our relationship with your Country,” Mr. Trump concluded.