Blue chip shares fell in afternoon trading as heavyweight oil stocks dipped after crude prices slumped below $50 a barrel for the first time this year.
The FTSE 100 Index was down 43.2 points to 7291.4, after the key US measure, West Texas Intermediate, slipped by more than 1% to $49.76 for the first time since December. Oil prices are seen as a barometer of the health of world trade.
The UK oil measure, Brent Crude, fell by 0.9% to $52.64, while the FTSE 250 was 73.3 points lower at 18857.8.
IG chief market analyst Chris Beauchamp said "falling commodity prices are taking their toll".
Royal Dutch Shell was one of the biggest fallers in the top flight, down 61p to 2164.5p, despite announcing a large sale that takes it two-third of the way towards its disposal programme target.
The Anglo-Dutch oil giant said it will sell significant parts of its oil sands sites in Canada for $7.3bn (£6bn), as it continues to streamline the group.
It said it would sell down its share in the Athabasca Oil Sands Project from 60% to 10% to a unit of Canadian Natural Resources, one of the largest independent crude oil and natural gas producers in the world.
A second deal will see Shell and Canadian Natural Resources jointly acquire Marathon Oil Canada, which holds a 20% interest in the Athabasca project, for $1.25bn each.
The move is part of a divestment plan following Shell's 2015 mega-takeover of gas giant BG for $47bn.
Analysts at RBC said: "This is the largest divestment announced to date under Shell's $30bn divestment plan, which now stands at around $20bn."
BP was also 6.6p lower at 435.1p.
The biggest afternoon risers in the FTSE 100 Index were Aviva (+33p to 544p), easyJet (+36.5p to 976.5p), International Airlines Group (+20.5p to 568.5p), Admiral Group (+58p to 1887p) and Capita (+11p to 535.5p).
The biggest afternoon fallers in the FTSE 100 Index were Morrisons (-16p to 231.1p), BHP Billiton (-70.5p to 1255.5p), Anglo American (-48.5p to 1156p), Royal Dutch Shell (-61p to 2164.5p) and Antofagasta (-20.5p to 757.5p).
The biggest afternoon risers in the FTSE 250 Index were Dignity (+142p to 2438p), Cobham (+5.3p to 132.4p), Restaurant Group (+15p to 385.5p), Ladbrokes Coral Group (+3.4p to 124.5p) and Just Eat (+14p to 577p).
The biggest afternoon fallers in the FTSE 250 Index were Domino's Pizza (-54.3p to 339.7p), Ferrexpo (-10.2p to 141.4p), Kaz Minerals (-31.2p to 456p), Petra Diamonds (-8.7p to 133.5p) and Evraz (-13p to 211p).