“Before I start,” said Prince William, on his first official engagement in the tricky world of contemporary art, “I would like to admit something.”

The audience at the Whitechapel Gallery, where William was opening the newly expanded centre, must have wondered what was coming next. A confession that he did not really know the first thing about modern art? The announcement that he and Kate were going to get married? Something ghastly involving Damien Hirst?

“I am actually . . . Banksy,” the Prince said. “I thought that this would be a good forum to bring it out.” He paused, then added: “Any autographs I will do later.”

As ice-breakers go, William’s effort probably has to go down as an unqualified success. Not only did it show that he actually knew who the elusive guerrilla artist was, but it was also one of the better jokes told in recent years by a member of the Royal Family.

Not that it did him any good with Jumma Akther, however. While the rest of the contemporary art grandees seemed favourably impressed by their first encounter with a prince who is better known for flying helicopters and playing polo than for any deep-seated appreciation of cutting-edge art — the conceptual artist Michael Craig-Martin said he was “very charming, very amusing” — Jumma was less overwhelmed, which may have had something to do with her being eight years old.

She was one of a group of children from Biglands Green Primary School, Tower Hamlets, who were having an art lesson at the gallery when William called. She was painting a picture of Tower Bridge when the Prince passed by her table and offered to make a little addition of his own.

“It’s a window,” he said, painting a square in one of the towers. A quick Rolf Harris-style squiggle with the paintbrush, and then he added: “There’s a dog in the window.”

Asked later if she had known who that man was, she shook her head. What did she think of his work, then? She shook her head again. “There shouldn’t be a dog in there,” she said.

Even if he failed the practical the Prince — who studied history of art at St Andrews before switching to geography — passed the rest of his Whitechapel examination with, well, at least an upper second. It might be said, however, that when he chose to enter the armed forces, the art world did not lose one of its most insightful critics. “Whoa!” he said, as Bridget Riley showed him one of her wavey-stripey paintings in the British Council collection. “Did you get dizzy doing it?”

The artist Michael Craig-Martin showed him round the rest of the British Council works including a Damien Hirst spot painting and works by Lucian Freud, David Hockney and Gilbert and George.

“Because he studied art history you could tell he was very comfortable looking at the work,” Craig-Martin said. “He showed a natural interest in the things I was showing him.

“It can be difficult taking someone round an exhibition, but he expressed interest in these paintings both about the technical [aspects] and the subject matter . . . I never got the feeling that he was trying to escape.”

Of all the works there, however, William showed most interest in one that will not be gracing the Royal Collection for a while yet: Chris Ofili’s Painting With Shit On It, which is indeed a painting affixed with a large lump of elephant dung.

“He was very amused by that,” Craig-Martin said. “He said he liked it. I did point out to him that it was elephant turds. He was obviously quite taken with it.”

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