The heat wave began on Sunday and could "last nine or ten days, which would make it one of the three longest heat waves Spain has seen since 1975," Spain's meteorological agency AEMET says.
Temperatures have soared above 40 degrees Celsius across large parts of Spain and Portugal as the Iberian Peninsula faces a second heat wave in less than a month.
The mercury was to hit 42 degrees Celsius in Spain's southeastern region of Extremadura and 41 degrees in Andalusia, Spain's meteorological agency AEMET said on Monday.
Temperatures in Spain's normally cooler northwestern region of Galicia were predicted to rise above 35C.
"This heatwave really has the potential to be exceptional," said AEMET spokesman Ruben del Campo.
The heat wave began on Sunday and could "last nine or ten days, which would make it one of the three longest heat waves Spain has seen since 1975," he said.
Water reservoirs in Spain stood at 45.3 percent of capacity on Monday, well below the average of 65.7 percent recorded during this period over the past decade.
READ MORE: Spain battles wildfires amid record-breaking heatwave across Europe
June also saw Spain grapple with temperatures above 40 degrees Celsius in swathes of the country.
And the previous month was Spain's hottest May since the beginning of the century.
In August 2021 Spain recorded its highest ever temperature when the mercury reached 47.4C in the southern town of Montoro.
In neighbouring Portugal, the thermometer topped 44C over the weekend, fuelling wildfires and vast smoke clouds which were visible in the capital Lisbon.
Firefighters brought under control Monday the largest blaze which was burning in the central municipality of Ourem, local officials said.
While temperatures eased somewhat in Portugal on Monday they were expected to soar again in the coming days with 44C forecast for the southeastern city of Evora.
READ MORE: Officials warn of extreme fire risk as Europe swelters in June heatwave