Crown Prince Felipe, 46, will take over the Spanish throne following the abdication of his father, Juan Carlos I.
Spain's Prime Minister, Mariano Rajoy, said a new constitutional amendment would be proposed to allow Juan Carlos to abdicate.
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Her Majesty toured the show for 45 minutes.
M&S chief executive, Steve Rowe, has decided to take a more aggressive approach.
People are being encouraged to leave tributes and messages on the trees, which are on a trail in the city close to Manchester Arena.