In early trade, the FTSE 100 was 5.51 points lower at 6,815.28. Anglo American shares fell 2.1% while BHP Billiton dropped 1.9%.
Mining shares have been hit as prices of commodities have come under pressure from a recent strengthening in the value of the US dollar.
Morrisons fell 2.4% after Goldman Sachs cuts its rating on the firm to "sell".
Banking shares were having a better day, with HSBC up 1.4% and Barclays rising 0.5%.
On the currency markets, the pound fell 0.2% against the dollar to $1.3103 but rose 0.2% against the euro to â‚¬1.1757.
(Open): The UK's benchmark share index opened lower as shares in mining companies continued to slide.