Fed chairman Ben Bernanke said late on Wednesday the bank could scale back quantitative easing in coming months if economic momentum was maintained, thus threatening the removal of the stimulus which has been a key driver of the year-long equity market rally.
News of a slowdown in factory activity in China, a top market for Britain's heavyweight miners, gave investors a further excuse to take profits.
The FTSE 100 fell 143.48 points, or 2.1 percent, to 6,696.79, retreating from Wednesday's 13-year peak of 6,875.62 and suffering its biggest one-day drop since last May.
"Given the storming start we've had to the year, a bit of profit taking at some point was inevitable. Markets have reached year end levels that most people had penciled in ... and most institutional investors don't have a great deal of confidence in this market, nobody is in this for the long haul," said Mike Ingram, market analyst at BGC Partners.
Of the 100 blue chips, 94 closed lower - the highest number of fallers since September, according to Datastream.
There was some buying into the dip towards the end of the session, but the move was more muted than in previous falls.
"You would need to see several hundred points off the index before you can think about putting money in," said Andrew Feldhaus, investment manager at Redmayne Bentley.
Smaller caps, which tend to be more focused on company specific factors than macro news, fared a bit better, with the FTSE Small Caps index down 1.7 percent.
"Some of the smaller companies in the UK are still very under-owned, some of them continue to trade well irrespective of the economy at large and, if they've got a strong balance sheet,whilst their share price might go down, hopefully after it all settles down they will go back up again because they are already cheap," said Gervais Williams, managing director at Miton Group.
Within the bigger stocks, investors focused on companies with strong dividends. United Utilities was one of the few gainers, up 0.8 percent, after unveiling in-line full-year results and increasing its payout to shareholders.
Meanwhile mid-cap bicycles-to-car-parts group Halfords paid for slashing its dividend to fund a three-year sales push, sending shares down 16 percent.