The reception was the queen's first official event since she was hospitalized last week for gastroenteritis, the BBC reported. She did not attend the annual Commonwealth Day service at Westminster Abbey earlier in the day.
The charter was approved by the 54 Commonwealth countries in December. The queen called it a "significant milestone."
"We have now, for the first time, a single document that captures the core values and aspirations of the Commonwealth and all its members," she said.
The part of the charter that has gained the most attention appears to be an implicit endorsement of homosexual rights that does not mention homosexuality, which continues to be illegal in some Commonwealth countries.
"We are implacably opposed to all forms of discrimination, whether rooted in gender, race, color, creed, political belief or other grounds," the charter says.
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