Kerr-McGee, the US oil company, has been slashed to junk by all three major credit rating agencies after it bowed to pressure from corporate raider Carl Icahn and unveiled plans to buy back $4bn (€3.1bn) of shares.

Fitch reacted the most swiftly and aggressively after Kerr-McGee said yesterday it would repurchase between 27% and 29% of its 160 million shares, funding the deal through bank facilities arranged by JP Morgan and Lehman Brothers.