The events are one of the high points of the monarch's calendar for thousands of ordinary people across the country who are invited to attend.

Among the guests will be two of Britain's three remaining First World War veterans.

Henry Allingham, 111, the country's oldest man and a Royal Navy veteran who saw action at Ypres and the Somme will be joined by ex-Royal Navy stoker Bill Stone, 106, the last known ex-serviceman, living in Britain, to have served in both world wars.

Two great-great grandsons of soldiers who died in the First World War battle of Passchendeale have also been invited.

Rupert Forrester, 18, from Easkey, Ireland, is the descendent of Lieutenant Colonel Harry Moorhouse killed on the same day and within an hour of his son Captain Ronald Moorhouse in 1917.

Jacob Pratt, 13, from Otley in Leeds is the great-great grandson of Walter Wardman, also killed in the battle that claimed the lives of tens of thousands of men.

The teenagers will be taking part in a service in Belgium on Thursday, attended by the Queen, to mark the 90th anniversary of the battle. During the Queen's reign, more than 1.1 million people have attended garden parties at Buckingham Palace or the Palace of Holyroodhouse.

Guests consume more than 27,000 cups of tea at each event, as well as more than 20,000 sandwiches, in addition to a variety of sweet treats from 4,500 Dundee cakes to 8,000 chocolate cakes.

The two First World War veterans are also due to meet military chiefs at the Ministry of Defence on Tuesday.