Speaking on The Today Show in the US, Ferguson described the allegations, which have been strenuously denied by Buckingham Palace, that Prince Andrew had sex with underage "sex slave" Virginia Roberts as "salacious lies".
Ferguson described Prince Andrew, who she divorced in 1996, as "a great father and humongously good man".
Prince Andrew: Life in pictures
Prince Andrew: Life in pictures
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Prince Andrew, Duke of York arrives at the Royal Albert Hall on 8 November 2014 in London
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Prince Andrew, the Duke of York (L) speaks to Queen Elizabeth II on the Queens stand during Derby day at the Epsom Derby Festival, in Surrey on 1 June 2013
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Prince Andrew with his daughters Princess Eugenie and Princess Beatrice on the Balcony at Buckingham Palace, 2013
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Princess Beatrice (C) poses for photograph with her parents, Britain's Prince Andrew, the Duke York (L) and Sarah Ferguson following her graduation ceremony at Goldsmiths College, in London, 2011
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Prince Andrew’s ‘pointless plunge’ down the Shard was the moment to take stock of his usefulness
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Prince Andrew is the first royal to take and tweet a selfie
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Prince Andrew with Bahrain’s Crown Prince Salman bin Al-Khalifa at Royal Ascot in 2010
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Prince Andrew with Ilham Aliyev, president of Azerbaijan in 2009
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Prince Andrew with Indonesian President Susilo Bambang Yudhoyono on the first day of his state visit to London
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Duke of York in his uniform
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Britain's Prince Andrew speaks to the press during a meeting with Turkish Businessmen at Ciragan Palace in Istanbul on 26 May 2004
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Wearing a traditional Guatemalan ceremonial jacket, Britains Prince Andrew, the Duke of York, turns on the tap innaugurating an irrigation system sponsored by the European community on 7 March 2002 in Nevaj, Guatemala
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The newly wed Prince Andrew, the Duke of York and his wife Sarah Ferguson, the Duchess of York, wave to crowds on 23 July 1986 from the balcony of Buckingham Palace in London while Queen Elizabeth II and Queen Mother look on
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Prince Andrew and Prince Edward with their governess, Lavinia Keppel, at the Children's Book Show in Westminster, London on 7 November 1969
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Prince Andrew (bottom left) with his parents Queen Elizabeth II and Prince Philip and siblings, Prince Charles, Princess Anne, and little Prince Edward at Windsor Castle, 1965
"I will have not one word said about him on any level, any level," Ferguson said.
"I want to stand by him, because I know what it feels like to have salacious lies made up about you," she added. "I won't stand by and let his character be defamed to this level."
Ferguson has already spoken out strongly in her ex husband’s defence over the allegations.
Talking to reporters in Verbier, she said: "The York family is a tight unit. We've always been a tight unit.
"He is the greatest man there is. It was the finest moment of my life in 1986 when I married him. He is a great man, the best in the world."Reuse content