The decision, which relates to Prince Andrew’s “numerous” business trips between 2001 and 2011, means the files won’t be available for scrutiny during his lifetime.
The date was released as part of a response to a Freedom of Information request by royal biographer Andrew Lownie, who described the decision as “absurd”.
Prince Andrew, 63, was the UK’s special representative for trade and industry for ten years from 2001.
Mr Lownie told the Telegraph: “It is extraordinary that files relating to Prince Andrew, the subject of my next biography, will be closed until 2065.
“Many questions remain about his role as trade envoy, a public appointment paid for by the taxpayer, and his associations with figures such as Jeffrey Epstein.”
The Duke’s tenure ended in 2011 after he was forced to resign when a photograph emerged of him meeting Jeffrey Epstein in New York just after the financier had been released from jail after serving an 18-month sentence for sexual offences.
Under the Freedom of Information Act (2000), documents relating to the Royal household are exempt.
The Information Rights Unit of the Foreign Office said an exemption to releasing information relating to communications with, or on the behalf of, the late Queen, was “absolute”.
Mr Lownie said the UK needed a much more “grown up” approach to royal records. He claimed a delay in releasing information would create a “vacuum” for “fantasists”.
“Their release would go some way to restoring trust in institutions, not least the monarchy”, he added.