Royal & SunAlliance (R&SA) hit back angrily at ratings agency Moody's after its credit rating was downgraded to Baa2 from Baa3.
R&SA finance director Julian Hance denied the group's position had weakened since its last review by Moody's in April.
He said it had hit most of the major targets set in its November 2002 rescue plan including the IPO of its Australasian business and sale of RSUI. The group's capital position had improved by £900m.
He added: "We also reported improved trading results in the first quarter. We do not believe that there is any information that has been provided to Moody's which could have led it to this decision and, understandably, we are disappointed that our delivered improvements to date are not more fully taken into account."