The privatised postal group said parcels will be delivered on Sundays later in the summer to addresses within the M25 motorway, including London.
Around 100 delivery offices across the UK with the highest parcels volumes will be open on Sunday afternoons as part of the initiative. Royal Mail's delivery offices are currently open six days a week.
The firm's express parcels business, Parcelforce Worldwide, will also launch a Sunday delivery service in June for online shoppers through participating e-retailers. Parcelforce Worldwide will make the service available to contract customers across the UK.
Shoppers who choose the Sunday service through registered retailers will receive a text message between 30 and 90 minutes before delivery.
Royal Mail said the moves were being planned under the Agenda for Growth agreement with the Communication Workers Union (CWU).
Chief executive Moya Greene said: "Through these new Sunday services we are exploring ways to improve our flexibility and provide more options for people to receive items they have ordered online."