More than 20 towns in France have recorded their highest-ever May temperatures and the United Kingdom set a national heat record amid an extreme early-summer heat event that could mean the mercury climbs to 40 degrees in parts of Spain by the end of the week.The United Kingdom’s meteorological office said the country’s all-time record for May was broken when a temperature of 33.5 degrees was recorded at Heathrow near London, with highs of up to 35 degrees expected on Tuesday.Hundreds more May records are likely to be set in France, Spain and the UK, said forecasters, with temperatures exceeding norms by 12 or 13 degrees in what Météo France described as a “premature, remarkable and long” heat episode expected to last several more days.[ Ireland weather: All you need to knowOpens in new window ]France’s national weather agency said the record temperatures were caused by a heat dome, with hot air from Morocco trapped under an area of high pressure, adding that Europe could expect such events “more and more often, more and more intense, and earlier and earlier”.Tourists shelter from the hot weather beneath an umbrella on Westminster Bridge, central London. Photograph: James Manning/PA