Queen Elizabeth II is now considered to be the longest reigning monarch in British history.
She has surpassed her great-great-grandmother Queen Victoria's record on the throne and has reigned for more than 23,226 days, 16 hours and 30 minutes.
Speaking as she opened the Borders Railway in Tweedbank this morning, she said the milestone was "not one to which I have ever aspired".
She added: "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."
She is believed to have passed Victoria's record at around 5.30pm but the exact moment she became Queen is hard to calculate as her father George VI died in his sleep aged 56 in the early hours of the morning, possibly at around 1am.
The 89-year-old monarch, with the Duke of Edinburgh at her side, spent the day travelling on the railway with Scotland's First Minister Nicola Sturgeon, starting at Edinburgh's Waverley Station, stopping off at Newtongrange in Midlothian before carrying on to the Borders town of Tweedbank.
She delivered her short speech at her last stop, demonstrating her business as usual approach at the end by remarking: "So now to the business in hand. It is my very happy duty to declare the Borders Railway open."
There was no mention of Victoria in the Queen's address but she was notably wearing her diamond bow brooch, which originally belonged to her ancestor.
Normal service was suspended for a time in the House of Commons as MPs paid tribute, with Prime Minister David Cameron saying: "The Queen is our Queen and we could not be more proud of her.
"She has served this country with an unerring grace, dignity and decency and long may she continue to do so."
Queen Elizabeth II: In Pictures
Ms Sturgeon told the Queen in a speech: "For those watching from around the world let me say, on their behalf ma'am a simple but heartfelt thank you."
Afterwards, she described the Queen as being "in tremendous form".
The Queen’s visit to the Republic in 2011 was seen as a symbolic normalisation of relations with the United Kingdom following the signing of the Good Friday Agreement in 1998.
Britain's monarchy is now wealthier than ever, according to a Reuters analysis of royal assets, with nominal assets of about €32bn.
