The society says intermediaries will be able to obtain its KFIs in three ways: directly from its website; from the Mortgage Trading Exchange; and via other sourcing systems.
It says any KFIs sourced either from its site or from MTE will be guaranteed penny perfect.
It adds it is co-operating with other sourcing systems to provide product information so that they can produce Nationwide KFIs but the responsibility for making sure that the calculations are correct lies with the sourcing systems as it will not be guaranteeing them.
This clarification comes as the society last Friday announced it has started issuing KFI-style mortgage documents to branch and call centre customers in preparation.
It says the documents are similar to the official FSA KFIs required to be produced by all lenders under the new regulations.