The administrators of failed structured product provider NDF Administration has warned investors that it plans to keep the company in administration for another year.

Administrators Grant Thornton said it was continuing to negotiate a sale of the failed firm's ISA Mortgage Book, but believed it would be a 'protracted process involving contract negotiations with third-party insurers.'

ISA mortgages are similar to endowment mortgages, whereby mortgagees make interest payments to the provider and pay off the sum at the end of the term. Borrowers are expected to make regular savings in an ISA, with the gains used to pay off the mortgage.

Grant Thornton warned the complexity of administration would require considerable technical development to integrate the book into their business.

Regulatory permission may also be required of a potential buyer, which could take up to six months.

The administrator said NDF Administration retained Financial Services Authority permissions that could be lost if the company was wound up, and remove the mortgage book's realisable value.

The firm was put into administration in October 2009 and its investment books were acquired by Meteor Asset Management earlier this year.

The provider and sister company Defined Returns were put into administration after being put at risk of potential liabilities from Lehman-backed products. At the time, the administrators estimated that approximately 10 per cent of its 35,000 customers had invested in the Lehman-backed products.