NEW YORK (Associated Press) - Credit rating agency Standard & Poor's yesterday cut its long-term counterparty rating for Bermuda-based MF Global Ltd. because of a $141.5 million loss tied to unauthorised trading.

S&P cut the rating to investment-grade "BBB" from "BBB+". It also placed MF Global on negative credit watch, meaning there is a 50-percent chance it could be downgraded again in the next 90 days.

The Bermuda-based broker said on Wednesday morning it discovered Evan Dooley, a trader at the company's Memphis, Tennessee, branch, traded wheat contracts in amounts that exceeded how much he was allowed to trade.

MF Global said systems in place that should have blocked the trading failed. MF Global fired Dooley and liquidated the contracts, leading to a loss of $141.5 million.

Shares of MF Global fell $4.62, or 21.8 percent, to $16.57 in morning trading. Earlier in the session, shares hit a record low of $14. MF Global has been publicly traded since July 2007.