The London markets slid after traders were left largely unimpressed by the breadth of the new economic package announced by Chancellor Rishi Sunak.
A continued rise in case numbers in the UK weighed on sentiment at the start of the session, but the plans for a new Job Support Scheme did little to turn the tide.
Meanwhile, economists at JP Morgan were among those to warn that the latest economic package was comparably "small".
The FTSE 100 closed 76.48 points lower at 5,822.78 at the end of trading on Thursday.
Connor Campbell, financial analyst at Spreadex, said: "Disappointed by Rishi Sunak's Winter Economy Plan, yet alarmed by the need for it in the first place, the FTSE was easily the worst hit index on Thursday.
"Though there seemed to be a brief moment of respite around lunchtime, the European markets soon resumed the session's heavy losses."
The Eurozone indices were only marginally better as they saw sentiment drift after two broadly mixed trading days.
The German Dax decreased by 0.29%, while the French Cac moved 0.83% lower.
Across the Atlantic, the Dow Jones held off making any major moves, nudging only marginally higher after the opening bell.
Meanwhile, sterling made minor inroads on Thursday, stemming its recent slump against the US greenback.
The pound rose by 0.08% versus the US dollar at 1.274 and was up 0.42% against the euro at 1.092.
Travel firms made significant losses again as worries over the continuing spread of the virus hit the likes of Carnival and British Airways owner IAG.
In company news, Cineworld dived in value after it warned further global coronavirus restrictions or film delays may force it to raise further cash.
The group swung to a $1.6 billion (£1.3bn) loss for the six months to June 30 from pre-tax profits of $139.7 million (£110m) a year ago as revenues plummeted after lockdowns forced cinemas to close.
It saw shares drop by 7.16p to 41.36p at the close of play.
Sofa retailer DFS slipped after it swung to a statutory pre-tax loss of £74.9m for the year to June 28.
Shares fell by 7.6p to 160.4p despite reporting that the new financial year had started "very strongly" due to pent up demand.
Pets At Home surged higher after it said it expects to surpass its profit targets for the year due to strong recent momentum.
It closed 84.8p higher at 390p, after hailing double-digit percentage like-for-like customer sales growth over the eight weeks to September 10.
The price of oil drifted slightly lower as concerns over rising case numbers helped to drive fears that oil demand could wane.
The price of a barrel of Brent crude oil decreased by 0.53% to $41.54.
The biggest risers on the FTSE 100 were Pearson, up 31.4p at 536.6p, Persimmon, up 91p at 2,417p, Barratt, up 14.2p at 452.7p, and Polymetal, up 19p at 1,693.
The biggest fallers of the day were Rolls-Royce, down 12.3p at 150.1p, Smiths Group, down 107.5p at 1,324.5p, IAG, down 5.36p at 95.24p, and Hargreaves Lansdown, down 74.5p at 1,540.5p.
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