Prime Minister David Cameron will lead tributes to Britain's Queen Elizabeth II as she marks being the longest-serving monarch.

Queen Elizabeth

Queen Elizabeth

Later today (09.09.15) the queen will have reigned for 63 years and seven months, which amounts to 23,226 days, 16 hours and approximately 30 minutes and the Houses of Commons are postponing duties for 30 minutes to honour the moment in history and pay tribute to Elizabeth who David has described as a "rock of stability in a world of constant change".

The prime minister has posted a series of tweets about the queen and her "selfless" commitment to her role.

He tweeted: "It is only right that we should celebrate her extraordinary record, as well as the grace & dignity with which she serves our country (3/3) (sic)"

He wrote: "Her Majesty has been a rock of stability in a world of constant change, earning admiration for her selfless sense of service & duty (2/3) (sic)"

He added: "Millions of people across Britain will today mark the historic moment when Queen Elizabeth becomes our longest serving monarch (1/3) (sic)"

The 89-year-old royal will spend the day in Scotland where she will attend to her duties as usual including, the opening of a new railway line on the Scottish Borders Railway.

Back in London on the river Thames a flotilla of historic vessels will form a procession between Tower Bridge and the Houses of Parliament.

The bridge will be lifted as a sign of respect, HMS Belfast will sound a four-gun salute and the BT Tower in the centre of the capital will have a message rotating which reads "Long may she reign".