The Department told us it believes it has good controls throughout the contract life cycle to prevent and minimise fraud.48 It told us its use of ‘open-book’ contracts is a particular strength as this enables oversight of suppliers.49 The Department also told us that last year its Cost Assurance and Analysis Service disallowed some £400 million of potential supplier costs, which it noted are not captured in its reporting to the Public Sector Fraud Authority on its counter-fraud savings.50 After the session the Department wrote to us to explain that payments to suppliers are blocked automatically if they fail a three-way check between invoice, purchase order and confirmation of receipt. Invoices that cannot be corrected in a straightforward manner are formally rejected. Reasons for rejection can include lack of a valid purchase order and exceeding contractual limits, with the most common reason being contractor error. The Department also added that the rejection of an invoice does not imply fraud.51