The FTSE 100 gave up 0.4pc, as of 0835 GMT, as Washington's tariffs on steel and aluminium imports from key Latin American trading partners rekindled fears of trade tensions and overshadowed encouraging data from China and euro zone economies.
Trump also threatened to slap separate tariffs on France and the European Union, which dragged shares of blue-chip oil majors Shell and BP lower, while miners dropped 1.5pc.
The FTSE 250 edged 0.1pc lower though Centamin outperformed, surging almost 10pc after Canada's Endeavour Mining disclosed a 1.47 billion pound ($1.89 billion) proposal for the British firm.
Centamin shares were on track for their best day in six months.
The FTSE 100, which has lost 2.4pc in nearly a week, lagged the broader European benchmark, with shares of pharma group AstraZeneca and Durex condom maker Reckitt Benckiser slipping roughly 1pc each as the sterling strengthened.
The currency gained after a Kantar poll showed Prime Minister Boris Johnson's Conservative Party had widened its lead over the opposition Labour Party slightly over the last week.
The broader tone for Tuesday's trading session, however, was set by Trump's trifecta of tariff warnings.
"This sudden new belligerence from the U.S. appears to have caught markets on the hop," CMC Markets' Michael Hewson said.
Dealers opted for a risk-off approach, with investor nerves from Asia to Europe gnawing away over how or when the United States and China can agree a truce in their damaging trade war.
"It's a clear signal to Beijing that Trump is not shying away from tariffs ... if there is no phase-one deal, the tariffs will go up," Markets.com analyst Neil Wilson said.In other news-driven moves, Cineworld jumped 5pc.
Though the cinema chain warned on its annual trading performance, it said its acquisition of Regal Entertainment would now lead to more synergies than earlier expected.