Ben Affleck's four-year-old son, Samuel, caught a cold from Prince George when they went to the same play area.
The 'Accountant' star has three children - Violet, 10, Seraphina, seven, four-year-old Samuel - with estranged wife Jennifer Garner and when they visited London earlier this year, they enjoyed time at an indoor play area.
However, Ben was unaware that among the other kids there were Prince George, three, and his 18-month-old sister Princess Charlotte.
He said: "I noticed this weird vibe from the other grownups. They were all very well dressed and they had earpieces and I thought, 'For a kids' place this is tight security!' I was the very last person to realise that Prince George and Princess Charlotte were in there playing with my kid."
And just days later, little Sam came down with a cold, which Ben is blaming it on George.
He told 'The Graham Norton Show': "I can now tell [Sam] that he got a cold from the King of England!"
Although they had a great time in London, Ben says they weren't impressed when they went to see the famed changing of the guard.
He admitted: "Now if it was in America - and I hate to admit this - you'd get a VIP pass. You'd get backstage and they would accommodate you. So I got a tour guide thinking this guy will accommodate me but that wasn't the case!
"He took us to the middle of the street but we were so far away not only couldn't we see the guard, we couldn't see the fence! We were surrounded by 50,000 tourists looking for selfie opportunities and we never saw the guards change! It was a disaster!"