Amid warnings of price increases, Canada moved quickly to retaliate, China said it had planned countermeasures, and Mexico said it would soon unveil its response.

But Mr Trump’s most incendiary claims are reserved for China. The Communist Party, he says, “has subsidised and otherwise incentivised” Chinese firms to “export fentanyl and related precursor chemicals that are used to produce synthetic opioids sold illicitly in the United States”.

President Donald Trump said on Sunday the sweeping tariffs that he has imposed on Mexico, Canada and China may cause "some pain" for Americans, as Wall Street and the largest U.S. trading partners signaled hope that the trade war would not last long.