Queen Elizabeth's "official" birthday will be marked with a royal salute at midday.
The British monarch turns 91 today (21.04.17), and the King's Troop Royal Horse Artillery and the Honourable Artillery will sound their guns in her honour, while she is expected to celebrate her day in private at Windsor Castle.
The Kensington House Twitter page has posted a message for HRH, which reads: "Wishing Her Majesty The Queen a very Happy 91st Birthday today. #QueensBirthday #HappyBirthdayHerMajesty #Queenat91 (sic)"
Clarence House, the home of Prince Charles and his wife the Duchess of Cornwall have shared an old black and white photo of Elizabeth when she was 29 and her son Charles, who would have been just six years old, from 1952 to mark the occasion.
Alongside the picture posted on the micro-blogging site, they wrote: "To mark The Queen's 91st birthday, we are sharing this photo from 1952 of Her Majesty and The Prince of Wales #HappyBirthdayHerMajesty (sic)"
For her second celebration, Elizabeth will be joined by members of the royal family at the annual Trooping the Colour parade, which starts of at the royal residence and moves to The Mall and Horseguards' Parade and will be held on June 17.
She is also expected to make an appearance on the balcony at Buckingham Palace on the day.
Last year, for her milestone 90th birthday, the queen received gifts including a purple bag of salt from the British Virgin Islands whilst Withyham parish in East Sussex, England offered the monarch a padded union flag with the words "Now we are Ninety" on it.
A DVD titled 'Three Cheers for the Queen' was also included as a present, and the Royal Canadian Mounted Police gifted the Queen a horse called Sir John, whilst the Royal Cavalry of Oman presented the British royal with a bronze statue of 40 horses and a carriage.