Blimey - Global Warning

One of London's leading brokers has demanded that clients put up significantly more cash to cover derivative positions - a move which traders fear could result in millions of shares being dumped on the market today.

MF Global informed clients yesterday that the "margin" on contract for differences (CFDs) was increasing on certain stocks from 25pc to 90pc. The clients have been given until this morning to put up the extra cash or close positions.

Traders fear that the move could increase market volatility as clients - unable to find the cash or transfer their investments to other brokers - are forced to close their positions.

"You are going to see a lot of forced selling," said one leading London stockbroker.