LONDON (Reuters) - Spain's King Felipe and Queen Letizia begin a pomp-laden state visit to Britain on Wednesday (July 12) to cement ties that have been strained by questions over the post-Brexit future of Gibraltar, a British territory on the southern tip of Spain.

The Spanish royals will stay at Buckingham Palace during a visit that was twice postponed - once because of a political crisis in Spain in 2016 and then after it clashed with an early election Prime Minister Theresa May called in June.

Agreed before Britain voted to leave the European Union in June last year, the three-day Spanish state visit, the first in 31 years, now takes place just months after a long-running row over Gibraltar, the "Rock" captured by Britain in 1704 which Spain wants back, flared up again.

The EU and Britain have also yet to agree on guarantees for EU citizens living in Britain and British expats living in other EU countries. More than 300,000 Britons live in Spain.

The Spanish royals will have a private lunch at Buckingham Palace on Wednesday with Queen Elizabeth and her husband, Prince Philip, afternoon tea at the Prince of Wales' London residence Clarence House and a state banquet.

Felipe, who used a speech to the United Nations to invite Britain "to end the colonial anachronism of Gibraltar", will also address both houses of parliament on Wednesday and meet May at her Downing Street office on Thursday.

The future of Gibraltar, with a population of 30,000, is set to be a major point of contention in the Brexit talks. The EU annoyed Britain and Gibraltar in April by suggesting Spain have a right of veto over the territory's post-Brexit relationship with the bloc.