Queen Sofia of Spain will not be attending a lunch to celebrate the Queen's Diamond Jubilee amid a row over Gibraltar, it has been reported.
The Spanish government ordered the royal to decline an invitation to Friday's event at Windsor Castle.
El Pais newspaper in Spain said the snub was in response to a forthcoming trip to Gibraltar by Prince Edward.
Earlier this year Spanish prime minister Mariano Rajoy raised Madrid's long-standing demand for the return of Gibraltar during talks with David Cameron.
But Cameron told him that there was no change in the UK's position that the Mediterranean outpost should remain British.
Buckingham Palace has not published a list of invitees to the lunch and a spokesman declined to comment.
The event has already triggered protests as it is expected to include some controversial monarchs.
The lunch at Windsor Castle, hosted by the Queen, will also be attended by the Duke and Duchess of Cambridge, Prince Harry, the Duke of York and Princesses Beatrice and Eugenie, and the Earl and Countess of Wessex.
Guests are expected to include Swaziland's King Mswati III, who is accused of leading a lavish lifestyle while his people go hungry.
Another controversial figure expected to attend is Bahrain's King Hamad Al-Khalifa, whose country is in a state of civil unrest.
Yesterday, 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.
Swaziland Vigil, which organised the protest, stages regular demonstrations outside the Swaziland High Commission in London.
Co-ordinator Thobile Gwebu said people in the country had been reduced to eating cow dung to make sure they filled their stomachs when taking Aids medicines provided by aid agencies.
Gwebu said Swazis in the UK did not want to spoil the Queen's Jubilee celebrations and had written to her, asking her to influence the king.
The King of Bahrain's presence at the lunch is also expected to prompt controversy after pro-democracy protesters in the country were met with violent reprisals from police.
Last month, organisers of the Bahrain Grand Prix were urged to cancel the race amid concerns of human rights abuses, but it went ahead.
Bahrain's crown prince, Salman bin Hamad Al Khalifa, pulled out of the Duke and Duchess of Cambridge's wedding last year amid controversy over human rights.
Campaign groups compared the visit by the king to a visit from Libyan and Iraqi dictators Colonel Gaddafi or Saddam Hussein.
"What would the British citizens feel if Gaddafi or Saddam was invited for the Queen's lunch?," said a spokesman.