Customers pay for their postage on the company's website and then print a unique barcode directly onto envelopes, labels, or paper before the letter or parcel is posted.
Stamp collectors are disappointed that the black and white barcode will not feature the traditional image of the Queen.
The monarch's head has been a feature on British stamps since the Penny Black - the world's first adhesive stamp - was introduced in 1840, and because of this Britain is the only country in the world which does not have to include its name on the stamp
Michael Pitt-Payne, a fellow of the Royal Philatelic Society, said: "It will not have the same aesthetic appeal. You're losing some of the inherent qualities of stamps, the Queen's head and the colour, for instance."
John Moody, of Gibbons Stamp Monthly Magazine, said: "This is the beginning of the end for stamps, but it'll be a long time before we see post offices stopping selling them."
Royal Mail lost its monopoly on deliveries on January 1, and the new service is part of its fight against competition for business.
The company says the service was designed to give customers more flexibility and allowed people to use their services 24 hours a day, seven days a week.
Alex Batchelor, Royal Mail's marketing director, said: "The service is perfect for busy people, but it is also aimed at home workers who post small quantities of mail on a regular basis as well as people who sell goods via web auction sites.''