Under the arrangement, private firm UK Mail, a subsidiary of Business Post, will pay Royal Mail 13p a letter to deliver mail to business customers from April.
The deal follows controversial plans drawn up by the regulator to end Royal Mail's monopoly on collecting letters. There were initial concerns that the price for delivering letters for rivals would be too low or that other firms would "cream skim" lucrative parts of the business. However Royal Mail has now reached the agreement without any intervention from the regulator.
From April UK Mail will collect batches of 4,000 letters or more from business customers, sort them and then hand them to Royal Mail whose staff will deliver them to households.
Adam Crozier, Royal Mail's chief executive, said that it was a landmark deal, which would provide a commercial income without weakening the network.
The agreement is expected to be followed by several other similar deals after Crozier announced that he is ready to discuss contracts with other private companies.
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