The loss compares to an operating profit of £136m in the previous year.
Royal Mail said the universal service made a loss for the first time, of around £100m, while the overall price controlled area of Royal Mail's business made a loss of around £200m.
The group overall made a post-tax profit of £135m, down 53% from the previous year. Revenues were £9.2bn, up 2.3%.
Adam Crozier, chief executive of Royal Mail, said: "The Universal Service is a huge asset for Royal Mail and for the nation as a whole. It is part of the fabric of our society and is vitally important both to social cohesion and to the UK economy.
"But it is now in the red for the first time -- making an estimated loss of around £100m last year while the overall price controlled area of our business made an estimated loss of around £200m.
"So it is vital that we have the opportunity for a real debate about how the Universal Service should be financed and sustained going forward. We welcome the interim report of the independent review and share the belief as set out in the report that the Universal Service must be sustainable, and that competition should benefit all customers. We have a vision of how that can be achieved."