The company added that homeowners should not use energy until they are told it is safe to do so.
John Baker, Nationwide's head of insurance, said: "The terrible flooding we have seen across the UK has shown just how important it is to have adequate home buildings and contents insurance."
He added that his company would do "everything it can" to help its policyholders who have been affected.
Motorists in the UK were recently warned by Tesco Car Insurance that modifying their vehicles could void cover policies.
Research by the company found that Norwich is the place where the act of altering cars is most common.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.