Research by mailing firm Corporate Mailing Matters revealed that a 600m gap is set to open in Royal Mail revenues over the next three years, as businesses move to other postal operators.
The firm polled the UK's top 300 business mailers to find out how much mail they thought would move from Royal Mail to other providers over the next three years.
On average, those surveyed predicted a shift of almost 17% after full deregulation of the UK's postal services in January next year.
This will open up the competitive market to items costing less than 1 to send, and weighing less than 350g.
Referred to as the "licensed market", postal services regulator Postcomm said it was worth in the region 4.7bn in 2004/05, with business mail making up around 80%.
Postcomm has issued long-term licences to 14 providers.