The Duke of Edinburgh asked Scottish Conservative leader Annabel Goldie if she wore tartan knickers at today's papal reception, she revealed tonight.
The exchange was laughed off by Miss Goldie, who said afterwards it was good to know the royals have a "sense of humour".
It happened after the Pontiff was welcomed to the UK by the Queen at the Palace of Holyroodhouse in Edinburgh this morning.
They were then introduced to a group of dignitaries at an official reception, including several Scottish political leaders.
Prince Philip met Scottish Labour leader Iain Gray, who was believed to be sporting a special papal tartan tie.
At this point, the prince was said to have turned to Miss Goldie, who was standing nearby, and asked if she had a "pair of knickers made out of this?"
Miss Goldie retorted: "I couldn't possibly comment - and even if I did, I couldn't possibly exhibit them."
A Tory insider said afterwards that Miss Goldie was in "hoots of laughter" over the exchange.
She said: "It's marvellous to know that humour is alive and well in the royal family."
A royal spokeswoman said tonight: "We don't comment on private conversations and this would come into that bracket."
Deputy Prime Minister Nick Clegg was among those at the event, as well as Scottish First Minister Alex Salmond.