Royal Mail has described , which has refused to comment on its reasons for moving the account away from the company, as "critical" to its business and that of rivals. It is not yet known where the business is moving to.
A spokeswoman for TNT Mail said she could not comment as to whether it was in the frame for Amazon's account, while another rival postal operator, UK Mail, said it had no link to the business.
The loss of the account is a blow to Royal Mail, which has cited higher costs as a direct reason for not being able to modernise operations and retain customers.
Royal Mail also said that disruption caused by strikes was undermining its ability to modernise and attract new business, describing the Amazon account as "an important area of growth" in an otherwise declining market.
Royal Mail, in a statement, said: "We're very proud to work with Amazon and the loss of such a significant piece of their business demonstrates very clearly that Royal Mail's higher costs, directly caused by our failure to fully modernise our operations, are costing us business.
"At the same time, our customers are being threatened with disruption because of strikes -- strikes which are aimed at preventing exactly the modernisation that could keep our big customers on board."
Earlier this month, Royal Mail union members voted for industrial action after rejecting the company's 2.5% pay rise offer.
However the Communications Worker's Union has not yet set a date for a strike and yesterday it agreed to hold talks with Royal Mail, which has said it will not improve its pay offer, at conciliation service Acas.