David Cameron will lead an hour-long tribute to the Queen in Parliament tonight as part of a worldwide wave of praise for her record-breaking reign.
The Prime Minister will be joined by the leaders of the other political parties in thanking Her Majesty for her 63 years of service on the day she overtakes Queen Victoria as our longest-reigning monarch.
National leaders from Commonwealth realms around the world are also expected to add their own words of thanks on a day of celebration that will begin on the Pacific island of Tuvalu and end in Canada.
The Queen is expected to respond to some of the messages if she makes a speech during the opening of a new railway line in the Borders at lunchtime, but she will not by then have heard the tributes from the Commons, which will begin with a statement from the Speaker, John Bercow.
Mr Cameron will stay in his seat after his weekly Prime Minister's Questions session with MPs before delivering his tribute to the Queen, followed by Harriet Harman, the interim Labour leader.
The messages from Commonwealth realms and nations will mean at least as much to the Queen as those that come from the UK, as she regards the organisation as her personal legacy. It had just eight members when the Queen became its head in 1952 - it now has 53.
Kamalesh Sharma, the Commonwealth secretary-general, paid an early tribute yesterday: "The Queen's personal commitment as head of the Commonwealth is exemplary, and her devotion to advancing co-operation and understanding continues to inspire people of all ages," he said.
Tuvalu is expected to be the first country to pay tribute followed by New Zealand, the Solomon Islands, Australia and Papua New Guinea.
After David Cameron's message from Parliament, attention will turn to Jamaica, Belize and the Caribbean island nations that have the Queen as their head of state, and to Canada, which has organised a half-hour musical tribute.