After an extremely long day of performing on Good Morning America, promoting her new album and lighting the Rockefeller Center Christmas tree, Britney was ready to unwind and celebrate her 27th birthday.

She celebrated her biggest birthday yet at Club Tenjune in New York.

Brit wore a posh low-cut pewter colored cocktail dress, but struggled to keep her bosom in check. The singer was frequently adjusting straps and tugging at the dress to keep the “twins” covered.

But still, a nipple did manage to slip out.

Britney’s funbags have minds of their own – and ‘lefty’ decided to catch a glimpse of all the excitement at one point.

Despite the wardrobe malfunction, Britney looked damn good. She ditched her usual ratty extensions and opted for a swept up hairstyle which framed her face and accented her flawless makeup.

See the nip slip below.

Below: Britney warms up in a winter coat at the Christmas tree lighting.