The King of Spain has said any Brexit deal must give the thousands of his compatriots in Britain, and UK expats in his country, "sufficient assurance and certainty" over their futures.
King Felipe said those citizens have a "legitimate expectation of decent and stable living conditions", as European Commission chief negotiator Michel Barnier again criticised the offer made by Prime Minister Theresa May on EU nationals living in the UK.
Addressing MPs and peers in Parliament’s Royal Gallery, the King said Brexit “saddens” Spain but that it "fully respects” the will of the British people.
The King went on: “At the bilateral level, our governments must work with determination to maintain and even improve the network of relations we have built within the European project.
“This is something that our societies - which are so profoundly intertwined - truly demand of us.
“Given these circumstances, we must particularly bear in mind the thousands of Britons and Spaniards who live in each of our nations, who form a sound foundation for our relations.
“Those citizens have a legitimate expectation of decent and stable living conditions, for themselves and their families.
“I therefore urge our two governments to continue working to ensure that the agreement on the UK’s withdrawal from the EU provides sufficient assurance and certainty.”
The King's speech, watched by Mrs May and Labour leader Jeremy Corbyn, came after Mr Barnier said the PM's offer on citizens' rights "doesn't enable those persons concerned to continue to live their lives as they do today".
Mrs May's package would not deliver reciprocal rights to those enjoyed by British expats in the other 27 EU nations on issues including the ability to keep families together, he said in Brussels.
Britain's refusal to accept the jurisdiction of the European Court of Justice "creates uncertainty", Mr Barnier added.
Meanwhile, King Felipe also highlighted the "key role" Spanish companies play in the British economy.
"The extremely rich relationship between our societies is matched by very rewarding economic and trade ties," he said.
"Spain is one of Britain's primary export markets, and the United Kingdom is the second-largest investor in Spain.
"In turn, the United Kingdom is the primary destination for Spain's foreign investment, and the third most important export market in the world for Spanish goods and services.
"Whether they operate in the banking industry, finance, transport, energy, construction, agriculture, technology or communications, Spanish companies, whatever their size, play a key role in the British economy.
"They all contribute to the growth of this nation, generating wealth and creating thousands of jobs."
King Felipe also praised the "dignity" shown by the British people during the recent spate of terror attacks.
He honoured the memory of Jo Cox, the Labour MP "savagely murdered" last year, and paid tribute to other victims of attacks and their families.
The King singled out Pc Keith Palmer, who died defending Parliament during March's Westminster terror attack, and Spanish bank worker Ignacio Echeverria, who died trying to save a woman from an attacker at London Bridge.
"I take this opportunity, in this House, to convey my solidarity with the British people, and the solidarity of all of Spain," he said.
"You can be sure of the unwavering affection of the Spanish people. We are your loyal friends and allies. Every craven terrorist attack is an attack on every one of us because the victims belong to all of us, no matter their origin, race, religion or condition.
"We owe them and their families our utmost respect, gratitude and unswerving determination."