UK shares dropped on Friday, with both the blue-chip and midcap indexes set for weekly declines, as growing uncertainty around U.S. trade policy kept investors cautious on equities.

The S&P 500 is on track for its worst week since September, the latter since November. Both are poised to fall for three weeks in a row, the worst run for either since August. In that three-week period, the tech-heavy Nasdaq has dropped nearly 9%, wiping out just shy of $2.8 trillion in market value in the process.