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.

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"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.

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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."

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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."

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