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:
Oh it's unbelievable, it's unbelievable.
I Just can't believe it's as long as Queen Victoria...such a big person in history, you know, to beat that and you know with such style.
The Royal Barge Gloriana is joining eight other vessels for the 'Royal River Salute' as part of commemorations on the day The Queen becomes Britain's longest-ever serving monarch.
Gloriana will be joined by police, fire, RNLI and commercial boats as part of the procession along the Thames from Tower Bridge to the Houses of Parliament.
HMS Belfast will also fire a 4-gun salute as the boats pass by.
The Queen has halted her journey on the new Scottish Borders Railway to meet well-wishers at Newtongrange in Midlothian.
The luxury steam locomotive, Union of South Africa, transporting the royal party is bound for the town of Tweedbank, where the opening ceremony of the £294 million railway will be held.
As the nation marks the occasion The Queen becomes the UK's longest reigning monarch, we take a look at some of the numbers behind her rule.Read the full story ›
Prime Minister David Cameron has paid fulsome tribute to the Queen on the day she becomes Britain's longest running monarch, describing her reign as a "golden thread running through three post-war generations".
Speaking in the House of Commons, Mr Cameron said: "I do think it's right that today we should stop and take a moment as a nation to stop and mark this historic milestone."
Her Majesty the Queen inspires us all with her remarkable service, her dignified leadership and the extraordinary grace with which she carries out her duties.
It is truly humbling to comprehend the scale of service that Her Majesty the Queen has given this country. The reign of Queen Elizabeth II has been a golden thread running through three post-war generations.
The Queen is our Queen and we couldn't be more proud of her. She has served this country with unerring grace, dignity and decency. And long may she continue to do so.
Her Majesty has arrived at Edinburgh Waverley station where she boarded a train to Tweedbank where she is expected to make a speech on the day she becomes the longest reigning British monarch.
The Queen is en route to Edinburgh Waverley Station to board a train with Scotland's First Minister Nicola Sturgeon that will head to the Borders town of Tweedbank where Her Majesty will officially open the new rail route.
Former prime minister Sir John Major has paid tribute to the Queen as an "absolute constant" in British public life.
Her Majesty will today surpass her great-great-grandmother Queen Victoria as Britain's longest reigning monarch after inheriting the throne in 1952.
Throughout the last 60 or so years, the Queen has been an absolute constant.
Whilst prime ministers have come and gone, celebrities have come and gone, life has changed, she and the monarchy have been an absolute constant in their lives and I think that is very reassuring.