The musician, who is expecting her second child with husband Sam Cooper, revealed on Twitter that her collapse in the luxury department store was down to an empty stomach.
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"Just fainted in Harrods. #croquemonsieur needed," she wrote on her @lilyrosecooper account. "On the bright side, managed to pick up [a Harry Potter] Professor Snape Wand in Ollivanders box and Slitherin pen for Nephews Xmas present. #luckyboy."
Allen's pregnancy was first confirmed by her father Keith Allen in July. She is expected to give birth next month.
The 'Smile' star welcomed a daughter named Ethel Mary last November, five months after she married Cooper.
In October, Allen weighed in on the debate over a woman's choice to have an abortion, slamming "small minded idiot blokes" and writing on Twitter: "The day the number of single father households equal the number of single mother households is the day I start to listen to their views."
