The Foreign Office has updated its travel advice for Spain to warn about the severe flooding which has seen the death toll rise to at least 95 people.
The Valencia region and Castilla La Mancha are among the areas that have been most affected by the torrential rain on Tuesday.
Torrential rain triggered flash floods which swept away bridges and buildings and forced people to climb on to roofs or cling to trees to survive.
Almost 50 flights using Valencia airport were cancelled and at least 30 diverted to other cities in Spain due to the floods.
In Chiva, near Valencia, more than a year’s worth of rain fell in an eight-hour period.
Spanish prime minister Pedro Sanchez declared three days of national mourning as the extreme conditions continue, restricting some rescue efforts.
The death toll is feared to climb as “there are many missing people”, the government says.
Updating its travel advice yesterday (Wednesday), the Foreign, Commonwealth & Development Office instructed UK travellers to check the latest weather warnings from Spain before starting their journeys.
The update said: “Severe weather and flooding is affecting many areas of southern and eastern Spain, particularly the Valencia region and Castilla La Mancha.
“Journeys may be affected. Check the latest weather warnings from Spain’s meteorological office before you travel and follow instructions from local authorities.”
The flooding, which has destroyed homes, streets and bridges, killed almost 100 people, according to authorities.
A British man died of hypothermia hours after being rescued from his home on the outskirts of Málaga, reported officials.
The Interior Ministry’s Integrated Operational Coordination Centre (Cecopi) said the death toll was provisional as it was still identifying bodies.
Around 155,000 households were left without electricity in the Valencia region, a local electricity provider reported.
Sanchez said: “I would like to, on behalf of the government and the whole of Spanish society, convey all of our solidarity and condolences to the families of those who have died in this tragedy in the region of Valencia and Castilla La Mancha.
“To those who are still looking for lost ones, Spain weeps with all of you – our absolute priority is to help you.”
The flooding is the deadliest to have hit Spain since 1996, when 87 people died in Biescas in the Pyrenees.
Reporting on this week’s flooding, BBC Weather’s Matt Taylor said extreme rainfall was becoming “more regular” as the climate warms.