Catherine, Princess of Wales giggled with Sir Paul McCartney over a joke about ageing

Touring the building with the likes of Sir Paul and artist Tracey Emin, 59, she was greeted by the singer with a handshake and introduced to Nancy - before Catherine laughed when 'Macca' - who has his own photographic exhibition opening there next week featuring images he personally took at the start of Beatlemania between 1963 and 1964 - joked about whether she or his wife would have even been born then.

Catherine wouldn't have been, but at 63, his wife reassured him she had.

The princess told Sir Paul: "Hello, nice to meet you too," before the 'Hey Jude' singer told her the staff at the National Portrait Gallery had been "amazing" and "great".

Catherine said: "It's amazing how long the planning is, how many conversations happen pre-everything."

Sir Paul added the contents of his exhibition were done "when you weren't even born," before he told his wife: "Oh, you weren't either."

Nancy looked up and counted before she replied she very much had been.

The group chatted in the gallery's gift shop and Catherine also asked Sir Paul how he had been able to choose "what pieces you wanted in there".

Princess Catherine, who is patron of the gallery, was also greeted by Jamie Fobert, the architect behind the project, and Tracey, who has created new artwork for the gallery's doors.

She also visited the Mildred and Simon Palley Learning Centre, where she joined a class of nursery children taking part in a Beatrix Potter-themed art class.