Queen Elizabeth II officially became the longest-reigning monarch in Britain's history on Wednesday, passing the record set by her great-great grandmother Queen Victoria, who served for 63 years and 216 days.

Despite the milestone, the queen and her husband, Prince Philip, conducted just one public event where she acknowledged the pivotal moment after reopening a restored railway line in Scotland.

"Prince Philip and I are very grateful for the warmth of your welcome," she told the jubilant crowd, reports NBC News.

"Many ... have also 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. Mine is no different. But I thank you all and the many others at home and overseas for you touching messages of great kindness."

According to NBC News contributor and royal expert Camilla Tominey, the queen didn't want to make a fuss about the significance of the day.

"Like Queen Victoria, Queen Elizabeth II hasn't wanted to make much of a fuss of this historic milestone. The present queen is much keener on celebrating her 90th birthday next year than this particular landmark," Tominey said.

Meanwhile, Prime Minister David Cameron described the queen as a "rock of stability" and declared that millions of Britons will celebrate the historic moment.

"Over the last 63 years, Her Majesty has been a rock of stability in a world of constant change and her selfless sense of service and duty has earned admiration not only in Britain, but right across the globe," Cameron said, reports The Associated Press.

"It is of course typical of her selfless sense of service that she would have us treat this day like any other. While I rarely advocate disobeying her majesty, I do think it is right that today we should stop and take a moment as a nation to mark this historic milestone," Cameron said in a speech to parliament on Wednesday. 

"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."

Elizabeth succeeded her father, King George VI, in 1952 at the age of 25 following his death. She has since reigned for 63 years and seven months, equivalent to 23,226 days, and 16 hours.