The deal is the first of its kind in Europe and affects business mail.
It enables Business Post's subsidiary UK Mail to access Royal Mail's local sorting and final delivery network.
From April, UK Mail will collect pre-sorted mail from its business customers and transport it to one of Royal Mail's 73 UK mail centres, before handing it over to Royal Mail for final sorting and delivery.
UK Mail will pay Royal Mail for the final sorting and delivery part of the process. The charge will depend on letter weight and the degree of final sorting necessary.
"The agreement finally gives business mailers a choice of delivery services," said Guy Buswell, managing director of UK Mail. "It also promises the first-ever business mail two-day guaranteed delivery service."
Letters weighing up to 60g will cost UK Mail 13p each, providing they have been sorted down to the level of local delivery office.
If letters require final sorting by Royal Mail, the price for letters less than an A4 size will be 13.375p, and 14p for A4 and over. Charges will rise for letters weighing more than 60g.
Under agreed delivery targets, UK Mail must hand mail over to Royal Mail the day after collecting it.
Ninety-five per cent of this must be delivered the day after it is handed to Royal Mail, to meet the service's two-day delivery specification.
The agreement comes after a long wait for UK Mail, which received its licence for business mail in 2001.
While it is the first company to benefit from an access agreement with Royal Mail, other companies will be able to establish an access operation under the same terms.