During her almost 60 year reign - Queen Elizabeth II has hosted hundreds of lunches for foreign monarchs.
Few in recent years have had such a controversial guest list.
The King of Bahrain, Hamad bin Isa Al Khalifa, is among those invited to dine at Windsor Castle at a time when Bahrain has come under international pressure over allegations of serious human rights violations.
The crackdown against pro-reform demonstrators has drawn disgust and concern in equal measure from campaigners.
Al Jazeera's Emma Hayward reports from London.