The London market slid despite shares in oil companies being boosted by a jump in oil prices.
The FTSE 100 started the day higher, but by the close, the index was down 63.79 points at 6,890.42.
The fall came despite oil majors BP and Royal Dutch Shell both rising by about 1.5% after crude prices surged.
Oil prices jumped more than 4% after non-Opec oil producing nations agreed to cut output in a deal designed to reduce oversupply and boost prices.
Opec announced last month that it would be cutting its own production.
Outside the energy sector, shares in Marks and Spencer rose 1.4% after Bank of America-Merrill Lynch upgraded its rating on the retailer to “buy”.
Shares in Sky slipped 2.8% after having surged on Friday, when it emerged that 21st Century Fox had made a takeover approach for the company.
Fox offered £10.75 a share for the 61% of the business it does not already own, valuing Sky at about £18.5bn.
But Sky’s shares fell 28p to 972p on Monday. Reports at the weekend suggested that some major shareholders were unhappy with the level of the offer.
On the currency markets, the pound rose 0.78% against the dollar to $1.2673 and gained 0.32% against the euro at €1.1947.