Charity workers flash a Christmas bra much to Prince Charles' delight after reception at Clarence House

But the Prince received a surprise himself when two of the female guests bared their bras to him.

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Surprise! Prince Charles showed what the charity event Walk the Walk really involves

Today's Clarence House reception marked 10 years since Ms Barough began the charity which has since raised more than £35 million for cancer care and research.

Each year thousands of people take part in its flagship fundraising event the Moonwalk.

This event sees walkers pounding the streets of London at night sporting their bras to raise money for charity.

And it was with this in mind that Julia Rennie, 36 and Julie Rogers, 28, both from South Wales, today opened their shirts to show the Prince their red fluffy bras decorated with golden crowns.

Mrs Rennie said: "The Prince stopped to chat and asked us how many events we had taken part in.

"I told him that I had to tell him a little story. I told him we had taken part in 19 events all wearing our bras. But the last event we took part in, in Portsmouth, we only wore t-shirts.

"When Nina saw us she told us that we couldn't be in her gang any more and the next time she saw us she wanted to see us in our bras.

"Then I told the Prince that I had something to show him and we showed him a little bit of our bras. Then someone told us we had to show the whole thing so we opened our shirts and showed him our bras. He was very interested in asking how they were made.

"I was very nervous doing it but I'm glad I did."

In a speech in which he thanked Ms Barough for her fundraising efforts, Charles referred to the women's spontaneous gesture.

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The Prince, who is patron of Walk The Walk, said: "Unfortunately I normally have to be with you in a virtual sense rather than in reality although Nina is always trying to get me to take part in one of these walks. Perhaps it would be handy if I could borrow one of these royal bras.

"Or maybe it would be worth doing it just to get a hug from Nina."

He added: "It is an extraordinary sight to see Nina completely speechless for once. She's the person who is the phenomenon and made all this happen and why you're all here today.

"For me it's been a great treat to meet all those who make everything happen behind the scenes."

The reception, organised by Breakthrough Breast Cancer and Penny Brohn Cancer Care, was attended by many of Ms Barough's family, friends, fundraisers and sponsors.

Ms Barough, who was only told about today's reception by her husband on her way to Clarence House this morning, greeted the Prince with an emotional hug.

She said she was "overwhelmed" by the surprise.

She said: "It's like you're getting married but nobody's told you that you're getting married.

"It's a wonderful surprise and it has been great to see so many people involved in Walk The Walk in one room. That normally only happens at occasions like weddings."

In a short speech at the end of the reception, she said: "I really am speechless. I just want to thank everybody here today; each one of you all know why you're here. You've all had a different part to play in Walk The Walk. I know Walk The Walk is very safe in all your hands."

Walk The Walk began a decade ago when 13 women power-walked the New York Marathon in their bras to raise money and awareness for breast cancer research.

From that first event more than £25,000 was raised for Breakthrough Breast Cancer.

Both this charity and Penny Brohn Cancer Care have benefited from the fundraising efforts.

The Prince is patron of all three charities involved in today's reception.