The Queen has said she had "never aspired" to become Britain's longest-reigning monarch but was "thankful" for the messages received on the historic day.
"A long life can pass by many milestones," she said as she opened a new railway line in Scotland. "My own is no exception."
She said she and the Duke of Edinburgh were "delighted to be back in the Borders" as she unveiled the new Scottish Borders Railway alongside Scotland's First Minister Nicola Sturgeon.
Many including you, First Minister, have kindly noted another significance attaching to today, although it is not one to which I have ever aspired.
Inevitably, a long life can pass by many milestones. My own is no exception.
But I thank you all, and the many others at home and overseas, for your touching messages of great kindness.
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