Britney Vs. Xtina! Spears 'Plans Retaliation' Against Aguilera For Mocking 'Voice' Skit

Before there was Kylie Jenner vs. Blac Chyna, there was Britney Spears vs. Christina Aguilera!

During the finale of The Voice, Aguilera did a spot-on impression of her lifelong nemesis Spears, in which she mocked the "Toxic" singer's intelligence, or supposed lack-there-of.

Now, almost three days later, a source close to Spears tells RadarOnline.com exclusively, "Britney thought that it was incredibly rude that Christina did that without running it by her first."

In the skit, Aguilera, 34, is dressed like Spears in a Catholic school girl uniform, her signature look from "Baby One More Time."

The controversial judge mocks Spears by saying such things as, "Join Team Britney, b*tch" and asks, "How do the singers know that you turned around if everybody's blind?"

Then, Aguilera took the drama one step further and off-screen when she congratulated Rumer Willis on her win for Dancing with the Stars. In a tweet, the "Dirrty" singer said, "Cool version of Toxic. And congrats on winning #DWTS girl! Xo," referring to an altered version of Spears' hit song.