The revised long-term credit rating for National Australia Bank of AA- may reveal it to be the most vulnerable of its domestic peer group, with Standard & Poor's citing its exposure to the stressed economies of Britain and New Zealand as factors that may lead to a further downgrade.S&P wrote that it had "minimal appetite at the current rating level for NAB experiencing material losses", a more severe form of words than used in the outlook summaries for any of the other three banks, ANZ, Commonwealth Bank or Westpac.The ratings agency wrote that it was continuing to "closely monitor NAB's UK strategy, including the potential for positive or negative credit developments."It said these were most likely to occur "should NAB eventually sell Clydesdale, which we independently believe is a possibility, or alternately that NAB will retain Clydesdale and potentially build upon its UK beach-head."In milder language, S&P also pointed out that for ANZ, "Prospects of positive ratings momentum in the short to medium term are unlikely".S&P said that to "maintain the stable outlook" for its credit rating it expected that ANZ's Asian regional strategy "will be well managed and not detract from the credit outlook".