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Sarah Brown wore a navy dress by designer Britt Lintner.
Brown and Obama visited Maggie's Cancer Caring Centre in West London. The first lady came bearing gifts: a book about the White House and a silver encased notepad and bookmark. The two women also spoke with patients and staff, and sat in on a make-up session for women suffering from cancer.
The First Lady shared beauty tips and described how her own make-up artist, whom she described as "wonderful", takes part in a similar programme.
Mrs Obama and Mrs Brown also had a cup of tea with staff and patients.
Introducing herself as "Michelle", she said she was planning a bee hive for the White House garden, joking: "We have a big lawn."
The British papers were impressed with Obama's physique. From the Daily Mail:
This morning's slim-fitting skirts had keen observers guessing if the US First Lady's toned physique was down to more than just hours in the gym.
A crease in the iridescent fabric halfway up Mrs Obama's right thigh looked remarkably similar to the VPL caused by so-called 'magic pants' - the body-slimming underwear made famous by Bridget Jones.