The 29-year-old singer wore a silver jumpsuit with a plunging neckline for the performance and accidentally flashed one of her nipples to the crowd. Allen had a similar nip slip when she wore the same jumpsuit to open Miley Cyrus' Bangerz tour earlier this year.
Reports say the audience did not seem to notice the slip as Allen recovered and continued the concert. The singer also slammed divorce rumors during the set, and dedicated a song to husband Sam Cooper.
"This song is for my husband, who I'm very much still with," she told the crowd. "So [expletive] anyone who says that I'm not."
Allen released her third studio album, Sheezus, in May. The record debuted at No. 1 on the U.K. Albums Chart, and includes the singles "Hard Out Here" and "Air Balloon."