The FTSE 100 dropped 37 points, or 0.5%, to 7,377 points by lunchtime, its lowest level since early October.
The biggest losers were mining firms including Glencore and Anglo American, along with oil giants BP and Shell.
It comes after further falls in global oil and commodity prices, with Brent crude extending its losses and falling more than 1% to $61.40 a barrel.
Oil prices also fell heavily on Tuesday after warnings from the International Energy Association of slowing demand and rising stockpiles.
Royal Dutch Shell shares were down 1.4% at £24.01, while BP was 1.2% lower at 496.95 pence.
Glencore, Rio Tinto and Anglo American were all down by more than 2% on the back of falling metal prices.
But gold miner Fresnillo was one of the biggest winners on the index – rising 1.8% – after gold prices edged 0.3% higher to $1,284.30 per ounce.