Queen Elizabeth II marked her 87th birthday with little fanfare on Sunday, spending the day privately while Prince William's wife Catherine was showing off her baby bump, AFP reports.
This year is an eventful one for the monarch as she will mark the 60th anniversary of her coronation, staged at Westminster Abbey on June 2 1953.
And her third great grandchild -- who will be directly in line to inherit the throne -- is expected to be born in mid-July to William and Kate.
The Duchess of Cambridge, 31, proudly displayed her growing bump beneath a pastel Mulberry coat as she stepped in for the queen at a function in Windsor Castle celebrating Britain's scout movement.
Traditional gun salutes were to be fired across the country, but military tributes will be held on Monday.
The ceremonial saluting battery of the Household Division will fire a 41-gun royal salute from London's Green Park while the Band of the Royal Artillery will play a range of celebratory music.
The queen's official birthday is in June, when Britain's weather is supposed to be more suitable for the grand Trooping of the Colour ceremony.
The queen has carried out more than 30,000 public engagements since her accession to the throne in 1952, according to avid royal follower Tim O'Donovan, 81.