The UK’s FTSE 100 exhibited a lackluster performance on Monday, as investors navigated a new wave of uncertainty surrounding U.S. trade policy. Meanwhile, Johnson Matthey experienced a decline following the acceptance of a significant reduction in the sale price of its catalyst unit to Honeywell. The blue-chip index was up 0.1% at 1200, following a week in which it closed at record highs. The domestically focused mid-cap FTSE 250 was down 0.1%.

The market exhibited volatility subsequent to U.S. President Donald Trump’s commitment to impose a new 15% tariff on global imports, in the wake of the U.S. Supreme Court’s ruling on Friday that annulled his extensive emergency-based tariffs. According to analysts, the tariff rate for the UK has risen from 10% to 15% in light of Trump’s most recent announcement. Shares of packaging and paper group Mondi fell 3.6%, while its U.S. peers also declined, impacted by renewed tariff concerns.

Johnson Matthey slumped 16.7% to the bottom of the mid-cap index after it agreed to slash the sale price of its catalyst technologies business to Honeywell, following underperformance in fiscal 2025. British aerospace and defence stocks pulled back after reaching a record high on Friday. Rolls-Royce Holdings opened new tab fell 0.7% despite a media report that the engine maker is expected to launch a share buyback worth as much as 1.5 billion pounds alongside its annual results this week.

Meanwhile, precious-metal miners rose 4% after gold climbed to a three-week high as investors sought safe-haven assets. On the political front, support for Prime Minister Keir Starmer’s Labour Party is waning in its Manchester stronghold, where the Green Party or right-wing Reform UK could disrupt decades of political tradition in an election later this month.