Barclays Bank has reported a £1.3 billion writedown on its exposure to the credit crisis. The Barclays Capital investment bank will write down £500m for the third quarter of 2007 and £800m for the month of October. The credit market turmoil has been sparked by the US sub-prime mortgage crisis. Barclays said the October writedown is a reflection of the impact of rating agency downgrades on a range of collaterised debt obligations which are backed by residential mortgages.