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Britney's new best friend London, escorted Brit to Ritual last night for Alli's birthday party! While Britney's cousin and closest pal has been MIA recently, the two were still up for getting down with a little b'day celebration where Britney introduced her new "baby" to Alli and the other guests.

An X17 source tells us the little pup, not unlike a baby, was up crying all night and found comfort only in Brit's arms. Awwwww!

And an even bigger scoop coming up on Monday -- you'll get a first listen at Britney's new single!!!!

more pix inside ...