Commodities and banks weighed on the FTSE 100 to send it down over 100 points to 6,199, off 1.7% by 8:55am.

Among the worst fallers were Anglo American and Glencore Xstrata, off 4.2% and 4.1% respectively, as fears about the slowdown in Chinese growth continue to hit commodity prices.

Barclays was the top faller – off 4.5% – after it was downgraded by Standard & Poor’s. Its credit rating was cut from A+ to A alongside two major banks in Europe: Deutsche and Credit Suisse.

The ratings agency said the banks faced increasing pressure from regulation and poor market conditions.

The falls came after a dismal session for Asia, with the Hong Kong Hang Seng off 2.6% and China’s Shanghai Shenzhen down 0.8%.

European shares were also tumbling as the eurozone crisis reared its head once again, this time in Portgual.

Yields on Portuguese debt have spiked in the last few weeks and are now at crisis levels above 8%, with the country’s stock market diving 6.5%, after the resignation this week of the country’s finance minister.

Across Europe shares are down, with France’s Cac 40 off 1.9% and the German DAX down 2.1%.