Theresa May has hailed the "deep and solid" ties between Britain and Spain as millions of pounds of business investment from the country is announced.
On the second day of a state visit by King Felipe, the Prime Minister insisted the two nations would maintain the "closest possible relationship" after Brexit.
As she hosts the Spanish monarch at No 10, Mrs May will pay tribute to Spanish banker Ignacio Echeverria, who died trying to save a woman from an attacker in the London Bridge terror attack.
"Britain's longstanding relationship with Spain has been built over centuries on the deep and solid foundations of our shared history, values and interests," the Prime Minister said.
"Today, we work closely together in a range of areas to ensure the security and prosperity of our people, including through our military and law enforcement cooperation to fight international terrorism, our academic collaboration on science and innovation, and our growing trade and investment ties.
"Indeed, the sheer scale of Spanish investment in Britain demonstrates Spain's continued confidence in the strength of the UK economy, and shows that we can and will maintain the closest possible relationship.
"This week's state visit is an opportunity to celebrate the historic bond between our countries and our citizens.
"We are firm friends, and I look forward to seeing our partnership go from strength to strength in the years ahead."
King Felipe and Queen Letizia's state visit is Spain's first since 1986 and comes as British and Spanish company bosses gather at the Mansion House for a business forum where they are due to be accompanied by the Duke of York.
Spanish manufacturer CAF is announcing that it will start building trains and trams at a new factory in Newport, South Wales, with £30 million of investment in the facility that will lead to 300 new jobs.
Other Spanish companies are boosting operations in the UK, including infrastructure company Sacyr, officials said.
International Trade Secretary Liam Fox said: "I'm delighted to welcome further Spanish investment into the UK today, as we remain Spain's number one destination in Europe for direct foreign investment.
"Our diplomatic relations stretch back more than 500 years, and as we leave the EU we want to remain a good friend and neighbour to Europe and build a positive new partnership with each of its member states."
The royal couple will also visit Westminster Abbey, accompanied by Prince Harry, before heading to the Spanish Ambassador's residence for a reception.