Sainsburys Bank Card: credit limit slashed?

Yes I have just received a letter today reducing my credit limit from �8300 to �100 with immediate effect. I opened the account in October 2004 and the card has been used regularly. I have never defaulted in any way but mostly pay off the card at the end of the month. I am outraged to be treated so poorly and will write to Sainsburys supermarket to tell them why I will never put my foot across their door again or buy a product sold by Sainsburys.