"We are delighted to have won a contract with one of the best known brands in the UK, with more than 6m customers this is a significant win," said TNT Mail UK managing director Nick Wells.

The deal is the first for TNT in the utility sector. Npower will use the firm's Premier 48-hour service, with Royal Mail undertaking the final sort and delivery.

"TNT has the size, scale and resources to deliver on what it promises and we are delighted to be working with it," said npower logistics manager Hilary O'Grady.

This win follows on from the 100m postal item contract with Lloyds TSB. Other clients include Sky and Caudwell Communications.

TNT Mail UK was launched in August 2004 in response to the deregulation of the postal market for mailings of more than 4,000 items.