Human rights activists have criticised the monarchy for allegedly inviting "blood-stained dictators" to the Queen's Diamond Jubilee lunch at Windsor Castle.
The king of Bahrain, Hamad Al-Khalifa, accused over a brutal crackdown on anti-regime and pro-democracy protesters, and Swaziland's King Mswati III are said to be invited to the castle on Friday. However the guest list has not yet officially been released.
Former foreign office minister Denis MacShane said many would "regret" a representative from Bahrain attending.
"Many in Britain will regret that the Foreign Secretary who approves all invitations sent in the Queen's name as head of state has decided to include a representative of the Bahraini regime which has done such terrible things to its own people since the Arab awakening a year ago.
"The FCO should protect the British Queen rather than expose her to having to dine with a despot."
Peter Tatchell decried any invitation for the Bahraini royals as "outrageous", telling the Huffington Post UK he expected eight dictators to be on the guestlist.
"Inviting blood-stained despots brings shame to our monarchy and tarnishes the Diamond Jubilee celebrations. It is a kick in the teeth to pro-democracy campaigners and political prisoners in these totalitarian royal regimes."
He told The Huffington Post UK: "Human rights defenders in Bahrain report that 50 civilians have been killed, with at least 1,500 arrested and another 1,500 wounded. Hundreds are in detention, including the country's leading humans rights activists. Saudi military forces are helping with this repression. For this reason, members of the Saudi royal family should not be on the guest list either."
Sarah Waldron of Campaign Against the Arms trade said the Queen's invitation risked condoning human rights abuses.
"This invitation, alongside the UK's continuing promotion and supply of military equipment to Bahrain, sends completely the wrong message - one of approval rather than condemnation of human rights abuses," she said in a statement to The Huffington Post UK.
The Duke and Duchess of Cambridge, Prince Harry, the Duke of York and Princesses Beatrice and Eugenie, and the Earl and Countess of Wessex are also expected to attend the lunch.
In 2011 the crown prince of Bahrain refused to attend the royal wedding after an international outcry.
On Wednesday a group of Swazis living in the UK protested outside London's luxurious Savoy Hotel, where King Mswati is said to be staying with an entourage of more than 30 people ahead of the Windsor gathering.
It was reported on Thursday Queen Sofia of Spain will not be attending the lunch amid a row over Gibraltar.
A Buckingham Palace spokesperson refused to comment on criticism, tellingThe Huffington Post UK the official guestlist would be released after midnight on Friday.
"These are official guests of the head of state tomorrow."