Moody's Investors Service said last night that it had placed the A2 long-term and P-1 short-term credit ratings of Clydesdale Bank under review for downgrade. Clydesdale is the main operating business of National Australia Bank in Britain.Moody's reduced the credit rating of Clydesdale as recently as December, while Fitch Ratings also cut the bank's credit rating in October.In December, Standard & Poor's cut the credit rating of Clydesdale Bank to BBB+ from A+.NAB has had to allocate around £1 billion in new capital to Clydesdale over the last two years, including £400 million in capital last month.