A secret letter written by Queen Elizabeth is locked inside a "vault" in a historic Sydney building and cannot be opened for another 63 years.

According to foreign media, the Telegraph reports, the handwritten note was completed in November 1986, following the restoration of the Queen Victoria Building in Sydney's CBD.

It is housed inside a glass case in a restricted area and no one knows what the letter says – not even the Queen's personal staff.

However, one thing is known – the letter cannot be opened until 2085.

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Addressed to the Mayor of Sydney, the instruction reads: "On a suitable day to be chosen by you in the year 2085 AD, please open this envelope and convey to the citizens of Sydney my message for them."

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She is simply signed, "Elizabeth R."

The Queen Victoria Building, or QVB as it is popularly known, opened in 1898. Queen Victoria was the great-grandmother of Queen Elizabeth.

Elizabeth, who became queen in 1952, visited Australia 16 times - the first time just two years after she was crowned.

She was just 27 - and the first sitting monarch to visit Australia.

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It was estimated that around one million people turned out to see him in Sydney.

Meanwhile, Elizabeth and her husband, Prince Philip, returned in 1963 and visited regularly over the following decades.

Her last visit to Australia was in 2011, when she met with then Prime Minister Julia Gillard.

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