Kate told a guest at yesterday's reception - where she and William cooed over and cuddled a newborn baby - that she wants her husband to play a trick on Prince Harry in revenge for the royal joker's live on-air comments that William and Kate would be entering next year's London marathon.
The prince was at the finishing line for Sunday's event to present trophies to the winners and when interviewed by BBC presenter Sue Barker told her: "What's fantastic is my brother and his wife will be doing it next year, I think."
When the host said his comments were "live on air," the royal replied "He's going to have to do it now, isn't he?"
But at yesterday's event, Kate said William would be looking to get back at his younger brother.
When chatting to one of the guests who had asked about the marathon the Duchess replied: "I think William's going to have to think of something to get him back."
But Kate, who is known to be a keen runner, did not rule out the couple taking part in next year's event during her conversation with John Ions, chief executive of Liontrust, an investment management company that sponsored the British soldiers who recreated the famous Scott-Amundsen race to the South Pole.
Mr Ions, who met the young Royals at the Goldsmiths' Hall in London, told the UK Press Association: "I mentioned to her that I had heard that she was doing the London Marathon. I had expected to hear that rebuffed but it seems it may be something they will do in due course."
Kate then seemed to volunteer her younger sister Pippa for the job, according to the businessman, who said she told him: "I'm more of a short-distance runner. My sister is more of a longer-distance runner."
Sixty years and hardly a slip.