The main index shed 0.7pc as financial stocks and oil firms dropped amid a fresh row between the two countries after US legislation on Hong Kong drew condemnation from Beijing, casting doubt on progress over the trade deal.
Shares in BAT outperformed, however, ahead of a meeting on vaping guidelines at the White House on Friday.
The mid-cap FTSE 250 slid 0.8pc, as Royal Mail's plunge more than offset a 6pc rise in Direct Line after Britain's biggest motor insurer laid out a plan to cut expenses.