Credit: Alex Moss/FilmMagic

The 2011 London holiday gala season may be short one major socialite: Pippa Middleton.

Duchess Kate's stylish, spotlight-stealing sister, 28, has cancelled plans to be among the high-profile revelers at the Square Mile Masked Ball Nov. 25.

"Pippa was set to attend the Square Mile Masked Ball on November 25th yes but sadly is no longer attending," a spokeswoman for the event tells Us Weekly.

"They heard from Pippa last week. She and a guest were due to attend but ball organisers heard the plans had changed on Thursday," an insider adds.

Perhaps Middleton doesn't know who to invite now as her plus-one. After all, about three weeks ago, she finally ended, for good, her romance with Alex Loudon.

"Pippa tried to get over the split by spending last weekend with [William and Kate] at Balmoral," a source told British paper The Sun. "She and Alex split briefly in the summer, but this time it's over. They are barely speaking."

In the wake of that painful split, the insider tells Us: "From what I hear Pippa's been making a few alterations to her social calendar. It looks like she's trying to keep a low profile -- which is a bit of a change."