Rome, Frosinone, Pescara and Campobasso were put on red alert on Tuesday, and they will be followed by Bari, Bologna, Bolzano, Brescia, Cagliari, Florence, Latina, Milan, Naples, Perugia, Rieti and Viterbo on Wednesday.
The addition of 10 other cities on Thursday will see all of Italy's major cities placed on red alert, except for the northern city of Genoa, which is on a less concerning yellow alert.
In a heatwave which meteorologists have described as the most intense of the last few years, record temperatures are being registered across the nation.
The apparent temperature reached 50˚C at Capo San Lorenzo on the southern island of Sardinia at 3:00 pm on Tuesday, according to the Italian air force's website.
The apparent temperature also hit 49˚C in Naples, 48˚C in Ferrara and 47˚C in Rimini.
Italy’s heatwave is expected to last until August 8.