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The Girls Aloud singer, who is based in LA, was due to fly into the UK for tonight's Earls Court show but ran into problems prior to departure.
A source said: "She got to the airport, realised she didn't have her passport and rushed back to her house to get it - but she couldn't find it."
"It's such a shame as it's the first time they've been nominated for a Brit and they were looking forward to attending together and having a great night," the source added.
Girls Aloud have been nominated in the Best British Group category along with Arctic Monkeys, Editors, Kaiser Chiefs and Take That.
