The Duchess of Cornwall, 71, 'informed the queen that she's happy to represent her at family occasions so she can rest and enjoy her twilight years,' says the insider, noting that she also encouraged the monarch, 92, to skip the christening in light of the long list of royal appointments she has coming up.
'Camilla insisted Kate would understand, and the queen believed her and was grateful for that.'
But, OK! claims Kate 'wasn't even informed that the queen wasn't coming until it was too late.'
While Kate, 36, didn't want to 'stir the pot' on the day of the christening, the source told OK! she's now ready to confront her stepmother-in-law.
'Kate's decided it's time to stop being so polite,' warns the insider. 'She's told William they need to fight back because they should be focusing on modernising the monarchy and ruling with integrity, which seems incapable of - and that this behaviour is proof of that.'
The royal family is yet to respond to OK!'s report.
Want to know what REALLY went on at Prince Louis' royal christening. Listen to the latest instalment of New Idea Royals.