National Australia Bank said a rating downgrade for its wholly-owned subsidiary Clydesdale Bank in Britain will not affect its own ratings.Moody's Investors Service downgraded Clydesdale's senior debt/deposit ratings to A1 negative outlook from Aa3 negative outlook.The bank financial strength rating was downgraded to C- with a stable outlook from C+ (with a negative outlook).Moody's said in a statement that the BFSR downgrade reflects the still very challenging economic environment in the United Kingdom and the impact that this is expected to have on the bank's profitability and asset quality.The ratings agency is reviewing all mortgage banks in Britain to take into account the deep recession and lower property prices.