UK stocks lagged their European peers and Wall Street, with the main index, which suffered its worst one-day drop since before the 2016 Brexit referendum on Wednesday, ending 0.6% lower. Energy stocks Shell and BP were the biggest drag as oil prices fell on the same worries about the global economy.
The mid-cap FTSE 250 slipped 0.7% to its lowest in more than a month after data showed the UK services sector unexpectedly shrank last month as the country's exit from the European Union dragged on.
On the blue-chip index, stocks trading ex-dividend added downward pressure, with packaging firm DS Smith, tobacco firm BAT, ad firm WPP, housebuilder Taylor Wimpey and retailer Kingfisher dropping between 2.4% and 5.7%.
A slew of shockingly weak US economic data, lingering fears over the Sino-US trade dispute and heightened no-deal Brexit jitters have spooked traders this week, and the World Trade Organization's approval of US tariffs on European goods has further fanned fears.
"With markets already looking vulnerable over concerns about a manufacturing recession starting to bleed into a slowdown in the services sector, the WTO ruling could not have come at a worse time," CMC Markets analyst Michael Hewson said. The globally-exposed FTSE 100 is headed for its steepest weekly fall in more than one-and-a-half years.
Notable movers included Ted Baker, which posted its biggest ever one-day decline, slumping 40% to a nine-year low after reporting a first-half pre-tax loss and warning that unseasonably warm weather, heavy discounting by rivals and weak consumer demand would hit full-year profit. Online trading platform CMC Markets climbed 7.7% after it forecast a jump in annual earnings.