Canada’s top trade negotiator has warned her American counterparts that if the White House imposes new tariffs on Aug. 19, the move would represent a “cliff” that risks halting negotiations with the U.S. According to five sources briefed on the discussions, Janice Charette delivered the message in a meeting last week with United States Trade Representative Jamieson Greer. Four of the sources said Ms. Charette cautioned the Americans on the public mood in Canada should President Donald Trump make good on his threat to impose the new tariffs. The sources said she warned the officials that Ottawa would not be able to rein in or control the public or premiers’ reactions and the federal government will have lost the room to negotiate with the Americans. The Globe and Mail is not identifying the sources, who were not authorized to disclose details of their private briefings on the sensitive trade talks. [A Globe and Mail subscription is required for full access]