Princess Anne will visit the RMS St Helena when it is moored alongside HMS Belfast as part of a farewell tour next month.
The cargo liner RMS St Helena has served as a lifeline for the British overseas territory of Saint Helena for the past quarter of a century, bringing passengers, goods and mail to the South Atlantic island.
The sea journey to Saint Helena takes about 14 days from the UK and about five from Cape Town in South Africa.
She is one of a handful of ships to have the designation Royal Mail Ship (RMS).
With the construction of a new airport, she has been withdrawn from service and put up for sale.
The ship will visit London and be moored alongside HMS Belfast for four days next month.
Tower Bridge will lift at 4.45pm on Tuesday 7 June for the arrival of the RMS St Helena and again at 5.45pm on Friday 10 June for her departure.
Princess Anne will attend a reception on board the vessel on Wednesday 8 June.
On Saturday 11 June the vessel will be open to the public at Tilbury.