The Monty Python legend spoke out via Twitter after reading a column Piers had written for Daily Mail's Event magazine in which he recounted a meeting between the men in New York.
Or lack of meeting, it seems.
Piers started his column quite nicely, saying he used to think John was a "comic genius" but he has "sadly morphed in recent years into the worst type of whining pub bore."
The America's Got Talent judge went on to say he encountered John in a NYC restaurant seated at the very next table.
"For one tiny nanosecond, our eyes locked in mutual shock, then equally mutual withering contempt. For the next two hours, we avoided all form of contact – physical, verbal, visual. It was magnificently British.
"Then I went to the loo, and returned to find my cousin Miranda in deep jocular conversation with him. They stopped the moment I sat down and the previous icy silence was resumed."
John's response? "Piers Morgan writes that I didn't recognise him in a restaurant in New York.I did.I just didn't want to speak to someone I truly detest."
Pies, however, hit back: "No, @JohnCleese, I said you DID recognise me & the revulsion was mutual. PS. when are you going to be funny again @JohnCleese? It's been a few decades."
For exclusive competitions and amazing prizes visit Reveal's Facebook page NOW!