The festivities will mark a rare joint appearance by the queen and the former Kate Middleton along with Camilla, the wife of Prince Charles.
The trio of senior royals are to meet military personnel at the landmark store Thursday and also will tour its famous food hall, long favored for its wines and delicacies.
The redone restaurant at the store has been renamed The Diamond Jubilee Tea Station to mark the queen's 60 years on the throne.
The royals plan to have tea with some of the store's staff, suppliers and former employees.
