Standard & Poor's cut its credit ratings for 15 banks including Barclays, HSBC, Royal Bank of Scotland and UBS AG, by one notch while updated its ratings for other banks as the top rating agency changed its criteria as a part of an overhaul to the company's rating system.

The new criteria focus on the strength of banks in the countries where they operate and will allow quick change in ratings and comparisons between banks.

S&P expects around 20% of banks would face a downgrade in their credit rating after applying the new criteria on their capital and other measurements, where the company noted that it would give a graduall update of 750 banks worldwide, starting with large banks.