The palace in a statement said the Queen will not attend the 200th anniversary of the Order of St Michael and St George because of her health.
Today's event was attended by American actress-director Angelina Jolie.
The Queen was due to attend a service at St Paul's today to mark the 200th anniversary of the Order of St Michael and St George, but was instead represented by the Duke of Kent.
On Tuesday night, Queen Elizabeth presented three Nigerians, Hauwa Ojeifo, Isaac Ezirim and Kennedy Ekezie-Joseph from Nigeria with a Queen's Young Leaders Award at a special ceremony at Buckingham Palace, London.
Last month she had eye surgery to remove a cataract. According to The Telegraph, no doctor was called to the palace and she was reportedly not transported to the hospital.
Israeli missiles hit near Damascus Airport: monitor
On Sunday, Israel struck positions of pro-government Shiite fighters near the Syrian-Iraqi border, killing over 40 of them. The U.N. said over 30,000 of the almost 50,000 displaced have headed to villages in the south near the border with Jordan.
The Queen is the longest serving monarch in British history, having marked 65 years on the throne last year - and she still maintains a busy schedule of events.
Prince Edward, Duke of Kent, is the queen's cousin.
"Members of the Royal Family will attend receptions at Buckingham Palace and a Horse Guards Parade for RAF servicemen and women, RAF veterans and representatives of RAF charities".
She was treated as a day patient and did not cancel any engagements nor appearances, but was spotted wearing sunglasses.