They sit alongside one another on the Britain's Got Talent judging panel.
But now David Walliams has threatened to boycott his pal Simon Cowell's 60th birthday bash.
David reportedly won't attend if controversial Good Morning Britain broadcaster Piers Morgan does.
Simon caused controversy recently when he cancelled his lavish 60th birthday party, just a day before it was due to take place.
Now it's been claimed that Simon has yet to set a date for the rearranged celebration due to David's threatened boycott.
Amanda Holden and Cheryl Cole are thought to be keen to attend Simon's bigger celebration and have already RSVP'd with a yes.
Poor Simon is stuck between a rock and a hard place.
A source told The Sun: "David refuses to be in the same room as his nemesis, and has told Simon it's him or Piers."
They added: "Poor Simon is stuck between a rock and a hard place. Piers and David may be ITV colleagues, but once the cameras stop rolling they cannot abide one another."
The source continued: "David hates Piers' tabloid newspaper background, and is fairly snobby about it, whilst Piers thinks David is a bit of a duplicitous [bleep] – nice to his face, and snide behind it."
Simon reportedly cancelled the original party to concentrate on work, including his TV show X Factor: Celebrity.
His spokesperson said: "Simon's priority is his incredibly busy work schedule over the next month and this, of course, includes the current series of The X Factor as well as other TV and music projects."
He did celebrate his 60th earlier in October with a small dinner with girlfriend Lauren Silverman in LA.
Head to our Facebook page @EntertainmentDailyFix and let us know what you think of our story.