A Foreign Office spokesman said: 'We are supporting the family of two British people following their deaths in Spain, and will do all we can to assist them at this deeply hard time.
Prime Minister Pedro Sanchez visited the rescue coordination centre and offered condolences for the victims.
The Britons, who were married and in their seventies, were travelling by taxi about 40 miles east of the Spanish island's capital, Palma, when the vehicle was swept away.
Two Britons are thought to have been killed during flash floods in Majorca.
At least five people were killed in flash flooding on the Spanish island of Mallorca late on Tuesday, according to police, with a British couple reportedly among the fatalities.
'Our staff remain in contact with the Spanish authorities who are responsible for responding to the floods, and are ready to assist any other British people who require our help'.
Spanish authorities said 80 soldiers would join more than 100 rescuers working in the area.
Video footage showed cars turned upside down, damaged houses and trees in the road.
At least three towns closed schools and hundreds of people were sheltered in sport facilities and a local horse racing track, authorities said.
"It all happened in less than 10 minutes", an unidentified witness with wet clothes and a blanket over his shoulders told TVE. The flooding also affected phone communications and forced the closure of at least seven major roads.
The Spanish weather agency said more heavy rainfall is expected on Wednesday in eastern and southern parts of the Iberian peninsula.
He was planning to travel to the area later on Wednesday, his office said.
Susan Guile and John Gray jetted out to the Balearic Islands on Sunday (7) for an 11-night holiday.
Heavy rain has also brought flooding to other parts of Spain, including Barcelona and the Costa del Sol.