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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.
Scottish National Party leader Nicola Sturgeon has paid tribute to the Queen during the inauguration of the new £294 million Scottish Borders Railway.
Ms Sturgeon said the Queen's reception showed the "strong admiration and affection" felt for her in Scotland.
"You have carried out your duties with dedication, wisdom and an exemplary sense of public service," she said.
"You are admired and held with great affection and esteem in the UK, across the Commonwealth and around the world."
This live broadcast has now ended.
Watch live footage from the Scottish town of Tweedbank where the Queen - who today becomes Britain's longest-reigning monarch - is expected to speak:
The Queen has resumed her train journey to Tweedbank as she marks her historic day by inaugurating the new £294 million Scottish Borders Railway.
Acting Labour leader Harriet Harman said the Queen is "admired by billions of people all around the world" in a tribute to her in the House of Commons.
Ms Harman praised Her Majesty's "tireless" commitment and dedication to the service of her country.
She also joked that the Queen had "gone from sending telegrams to sending tweets".
"We send her out warmest congratulations, our appreciation and above all our thanks," said Ms Harman.
The Duchess of Cornwall has praised Queen Elizabeth II's "style" on the day Her Majesty becomes Britain's longest reigning monarch.
Speaking to ITV News' Alastair Stewart, she said:
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