Media in the UK had reported that Miliband, a close ally of Secretary of State Hillary Clinton, would be offered the post.
The 45-year-old was narrowly beaten by younger brother Ed Miliband to become leader of the main opposition Labour Party following the organisation's defeat in a national election in May.
He remains a lawmaker at the House of Commons.
Miliband wrote on his Twitter account late yesterday that he was not in the running to become Britain's US ambassador.
Britain's Foreign Office said today that no decision has been made on replacing outgoing ambassador Nigel Sheinwald.
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