The King of Bahrain and Swaziland's King Mswati III are among controversial monarchs expected at a Windsor Castle lunch being hosted by the Queen.
Critics accuse Bahrain of human rights abuses and say King Mswati lives in luxury while his people starve.
Campaigner Peter Tatchell criticised The Queen for inviting "royal tyrants to celebrate her Diamond Jubilee".
Buckingham Palace said it will not comment on the lunch.
Saudi and Kuwaiti royals are also attending the banquet both countries and monarchs criticised for repression.
Meanwhile, Queen Sofia of Spain will not be attending because of a dispute over fishing rights off Gibraltar, a UK territory that Spain also claims.
Her husband, King Juan Carlos, had already declined an invitation because he is recovering from a broken hip.
Other members of the British royal family, including the Duke and Duchess of Cambridge, Prince Harry, the Earl and Countess of Wessex and the Duke of York and Princesses Beatrice and Eugenie will also are at the lunch.