The FTSE 100 ended 0.6 percent lower and the mid-cap index fell 0.8 percent.
Both indexes saw a small comeback around noon when pharmaceutical companies boosted the main index higher, after Bristol-Myers Squibb's $74 billion purchase of Celgene .
Pharma giants AstraZeneca and GlaxoSmithKline recouped earlier losses and rose 1.8 percent and 0.4 percent respectively, while a resurgence in crude prices helped oil majors eke out gains.
But Apple's comments on "economic deceleration" in Greater China plus data showing that US factory activity slowed more than expected in December ultimately had the final word.
Investors dumped stocks sensitive to China, the world's second-largest economy, and took refuge in gold, seen as a safe haven.
"What the market is wrestling with is whether it is indicative of a wider malaise in perhaps both the world economy and China," said Peter Rutter, head of global equities at Royal London Asset Management.
HSBC edged 0.8 percent lower and Standard Chartered lost 3.2 percent, while luxury brand Burberry, also sensitive to signs of slowing demand in China, lost 6 percent to join the top fallers.
Concerns over economic growth in top metals consumer China sent Rio Tinto, BHP, Glencore and Antofagasta down between 1.3 and 4.8 percent.
Blue-chip insurer Hiscox dipped nearly 5 percent after hacker/hacking group "The Dark Overlord" claimed to have stolen 18,000 files from UK insurance firms involved in litigation for the 9/11 attacks in New York.
A MIXED BAG OF NEWS
One bright spot helping contain negative sentiment was high street clothing retailer Next, which added 4.1 percent after reporting higher sales in the Christmas period thanks to a late surge in online demand.
"November was indeed difficult for Next as well, but Christmas did arrive ultimately, with the last three weeks of December being very strong in sales terms," said Peel Hunt analysts, while Investec called it a respectable trading update.
Next's encouraging update also helped shares in Marks & Spencer, Tesco, WM Morrison and Primark-owner Associated British Foods rise between 1.3 and 4.1 percent, among top blue-chip winners.
Prominent mid-cap and small-cap retailers, including Superdry, Dunelm, Debenhams, rose. AIM-listed ASOS was up 5.7 percent, also boosted by Peel Hunt reinstating a "buy" rating on the online fashion store a month after its profit alert shook the global retail scene.
Still, investors continued to fret about the US-China trade spat, a slowdown in the global economy, Brexit uncertainties, plunging oil prices - to name but a few.
Data showing growth in Britain's construction sector fell to a three-month low in December did little to help the mood, highlighting delays in commercial projects due to Britain's impending departure from the European Union.
Jitters over how Britain would end up leaving the European Union has also restricted spending by companies and consumers alike, as a mid-January parliamentary vote on Prime Minister Theresa May's disputed EU divorce deal looms.
Brexit minister Stephen Barclay said Britain was more likely to end up leaving the European Union without a deal if parliament rejects May's agreement.
Bank of England Governor Mark Carney warned in November that a "no deal" scenario would be an economic shock akin to the 1970s oil crisis.
Industrial and bank shares were among the top drags on the mid-cap index, and only consumer staples and real estate stocks ended in black.
Among small-caps, drugmaker Vectura soared 13 percent to lead the gainers after a positive trading update and AIM-listed Faroe Petroleum rose 4.9 percent after DNO's takeover offer became mandatory.