The Queen gifted King Felipe copies of love letters from his great-grandmother to King Alfonso XIII.
Letizia, who usually is spotted in Spanish designer labels like Felipe Varela, as well as their high street hitters Zara and Uterque, instead chose an all-yellow look more suited to a day out at Ascot for her visit, meeting The Queen, Prince Charles and the Duchess of Cornwall at Buckingham Palace.
The pair, who will meet at May's Downing Street residence on the second day of the three-day, pomp-laden state visit, are also likely to discuss the issue of European Union citizen's rights post-Brexit.
The trip is the first state visit by a Spanish king to the United Kingdom since Felipe's father, Juan Carlos, came 31 years ago.
Spain earlier this year agreed to the EU's ruling on Brexit negotiations that "no agreement between the European Union and the United Kingdom can be applied to the territory of Gibraltar without an agreement between the Kingdom of Spain and the United Kingdom".
Both royal lines are descended from Queen Victoria.
The wind died down and the sun broke through the clouds just as the Queen and Duke of Edinburgh stepped on to the dais at Horse Guards.
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Though with his retirement looming, this may unfortunately be the last time Prince Philip performs the ritual.
The King and Queen of Spain enjoyed a private lunch with the British Royal Family inside Buckingham Palace today - where they were served Laureate's Choice - a sherry selected by poet Carol Ann Duffy.
Image copyright EPA Image caption King Felipe inspected the guard of honour with Prince Philip. The Duke of Edinburgh and Queen Letizia travelled in a separate carriage. Prince Harry, Prince William and Kate Middleton are all expected to be in attendance.
A total of more than 1,000 troops are expected to take part in the state visit.
Felipe and Letizia are the first Spanish monarchs to visit Britain since Felipe's father toured the country in the 1980s.
At the United Nations a year ago, the Spanish King called on Britain to "end the colonial anachronism of Gibraltar".