Queen Elizabeth's special guests at Trooping the Colour

Queen Elizabeth has been joined by a select few at the modified Trooping the Colour celebration held on Saturday to officially mark her birthday.

Due to the coronavirus, the annual London parade was replaced with a pared-back ceremony on the grounds of Windsor Castle, where the Queen is in isolation.

Standing at Her Majesty's side during Saturday's ceremony at Windsor Castle was Vice-Admiral Tony Johnstone-Burt, the castle's Master of the Household, who has been managing staff during the coronavirus crisis on behalf of the Queen and Prince Philip.

Also spotted enjoying the toned-down event were staff members, who enjoyed the event through the windows of the residence, according to royal blogger Gert's Royals.

"Castle staff watch Trooping the Colours from the Windows of various buildings at Windsor Castle," they captioned some images on Twitter following the BBC's coverage of the event.

Her Majesty's birthday falls on April 21, but her "official" birthday has always been marked with the Trooping the Colour parade in June.

She is usually joined by other members of the royal family, who gather on the balcony of Buckingham Palace to watch the Royal Air Force flypast as well as the annual parade.

However, senior British royals including Prince Charles and the Duke and Duchess of Cambridge currently remain in isolation, despite some restrictions in the UK being lifted.

Trooping the Colour has marked the official birthday of the reigning monarch since 1748, with the 'colours' referring to the flags representing the different regiments of the British Army.

RELATED: Victoria Arbiter: This year's Trooping the Colour will be like no other

The event usually features hundreds of parading soldiers and horses, a carriage procession by the royal family, and a Royal Air Force flypast over Buckingham Palace.

On Saturday, the Queen was dressed in a Steward Parvin floral silk dress and mint coat, with a matching hat by Rachael Trevor-Morgan.

Her Majesty arrived at the Quadrangle of Windsor Castle and received a royal salute followed by military drills.

Soldiers from the 1^st Battalion Welsh Guards, who have been protecting the castle during lockdowns, marched around the grounds in accordance with social distancing rules. There was also music performed by a Band of the Household Division to close the ceremony.

This was the first time Her Majesty has appeared publicly since the beginning of the coronavirus pandemic.