The event was supposed to be a rare meeting of monarchs to celebrate the 60-year milestone, but it has been overshadowed by strong criticism from campaigners about those invited to the Windsor Castle event, including the King of Bahrain, King Hamad Al-Khalifa.
Guests from controversial regimes include Swaziland's King Mswati III, the former Prime Minister of Kuwait Sheikh Nasser Mohammad Al-Sabah, who stepped down over a corruption row, and Prince Mohammed Bin Nawaf Bin Abdulaziz Al Saud, Saudi Arabia's Ambassador to Britain.
Members of the British royal family attending the lunch included the Duke and Duchess of Cambridge, Prince Harry, the Duke of York and Princesses Beatrice and Eugenie and the Earl and Countess of Wessex.
The Queen hosted the 98 guests in Windsor Castle's St George' Hall.
The Queen, whose reign began in February 1952, is surrounded by her two cousins, who, had Communism not intervened, would have enjoyed even longer reigns. King Michael first ascended the Romanian throne in 1927, but three years later was displaced by hos own father King Carol II, who in 1940 packed his bags and left the country for good. King Michael's second reign this began in 1940 and by now he would have reigned longer than King Louis XIV of France, whose own reign lasted nearly seventy-two years. To Her Majesty's left side is King Simeon II of Bulgaria, who succeeded his father in 1943 and would celebrate next year seven decades on the throne had he not been forced out by Communists in 1946.