The 94-year-old monarch wore the mask during a memorial service in London earlier this week.
The event was held at Westminster Abbey before Sunday commemoration on November 11, the day the UK commemorates those who lost their lives in the war.
“The Queen marks the hundredth anniversary of the burial of the Unknown Warrior at Westminster Abbey this week, in a personal memorial service before Memorial Sunday,” Buckingham Palace said in a statement.
According to PA Media news agency, the black mask with white trim, is said to be led by Angela Kelly, the Queen’s private advisor and curator, who designs many of the king’s outfits.
The Tomb of the Unknown Warrior is the resting place of an unknown British soldier who died in World War I. The site has become a symbol of national mourning in the aftermath of the war and has retained its commemorative importance for many years since then.
In October, Queen Elizabeth appeared at her first royal engagement since the beginning of the pandemic but did not wear a mask despite a viral outbreak. In the UK, most indoor facilities are required to cover their faces.
She was photographed at the Porton Down science park in the south of England with her nephew Prince William, who was also not wearing a mask. Instructions for social distancing were given at the event and everyone the UK met tested negative for the virus.
Buckingham Palace said the Queen decided not to wear the mask after consulting with her own doctors and scientists at Porton Down.
The 71-year-old later said he had only mild symptoms.
It is unclear exactly when William was infected. The Sun, where the story was published, said he took a seven-day break from video calls and texts from April 9 to April 16.
The Queen’s nephew told an observer that he did not publish the results of his diagnosis for fear of people being worried. But his decision to keep matters private raises the question of why the British public is not aware of William’s illness.
The pandemic has swept over public life in England. British Prime Minister Boris Johnson also fell ill with the virus in the spring, while his top adviser Dominic Cummings had to go through a period of self-quarantine following symptoms.
The UK is currently in a second national lockout due to an increase in coronavirus infections.
Max Foster reports from London. Zamira Rahim writes from London.