As they await the arrival of their second child, Peter and Autumn Phillips enjoyed a rare night out with his grandmother, the Queen.
The royal party stepped out to see the hit comedy One Man, Two Guvnors at the Theatre Royal Haymarket.
The monarch and her grand children surprised theatre goers in the West End by arriving unannounced in Prince Philips' black London taxi cab which he uses for trips around the capital.
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They slipped in 10 minutes before the start of the show and sat in the circle stalls rather than the royal box.
"She was by all accounts having a fantastic time and laughing lots," a source told the Evening Standard.
"There is a particular part of a waiter who is very old and doddery, and she was visibly enjoying his scene.
"He falls down downstairs and his tray rattles when he is serving.
"The audience was greatly surprised. Everyone had their cameraphones out during the interval taking pictures."
Peter, the 34-year-old son of Princess Anne and her ex-husband Captain Mark Phillips, is the Queen's eldest grandchild.
His fairytale marriage to Autumn from Montreal, Canada in 2008 was followed by the birth of their daughter Savannah, the first great-grandchild for the monarch.