The Foreign Office has been accused of putting the Queen in the position of having to dine with despots as the row over a royal lunch with foreign monarchs intensified.
Denis MacShane, a former Foreign Office minister, criticised the guest list for apparently including the King of Bahrain, Hamad Al-Khalifa - whose regime brutally suppressed pro-democracy protests last year.
The Labour MP said many would regret Foreign Secretary William Hague's decision to approve the inclusion of the Middle East ruler.
His comments come after it emerged Queen Sophia of Spain was ordered by her government to turn down an invitation for the event at Windsor Castle to mark the Queen's Diamond Jubilee. The last-minute snub was in response to a reported trip the Earl of Wessex will make to Gibraltar, a UK overseas territory which Spain wants returned to its sovereignty.
Another controversial guest expected to attend is Swaziland's King Mswati III, who is accused of having a lavish lifestyle while his people starve. Protests were held outside the African ruler's exclusive London hotel on Wednesday where he is said to be staying with an entourage of more than 30 people.
Mr MacShane said: "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.
"Arab nations must let their citizens vote in free elections and let them speak without fear of arrest, torture or death. For too long we have turned a blind eye to the repression carried out under the rule of royals in Arabia. The FCO should protect the British Queen rather than expose her to having to dine with a despot."
Buckingham Palace has not commented on the guest list for the Windsor Palace lunch.
But the Royal Family have close links with many Middle East monarchs who are likely to attend and whose regimes have faced criticisms about human rights abuses.
Members of the British monarchy attending include the Duke and Duchess of Cambridge, Prince Harry, the Duke of York and princesses Beatrice and Eugenie and the Earl and Countess of Wessex.