Commodity-related shares dragged the benchmark UK share index lower, but there was better news for pharmaceuticals firm Shire.
Shortly after trading began the FTSE 100 index was down 28.61 points, or 0.4%, at 7,419.18.
Mining companies were among the biggest fallers, with Glencore and Anglo American both down by about 1.5%.
Shares in Royal Dutch Shell fell 1.3% and rival BP was also lower as oil prices remained below $45 a barrel.
Shire was the top riser in the FTSE 100, up 1.9% after the European Medicines Agency validated its marketing authorisation application for Veyvondi, which treats an inherited bleeding disorder called von Willebrand disease.
Outside the FTSE 100, shares in Imagination Technologies jumped 15% after the company put itself up for sale.
Imagination is in dispute with its largest customer, Apple, after the US tech giant announced earlier this year that it planned to stop using Imagination's technology in its products.
On the currency markets, the pound slipped 0.1% against the dollar to $1.2660 and was unchanged against the euro at 1.1350 euros.