There were 15 days where temperatures were registered above 25 Celsius (77 Fahrenheit) in the Nordic nation, part of which is situated above the Arctic Circle. The previous record in Finland was set in 2018, when 14 days were above 25C.

"It is very exceptional that we have had so many hot days already in May", climate researcher Mika Rantanen from the Finnish Meteorological Institute told AFP.

He said temperatures were forecasted to remain above 25C until the end of the month on Friday, which would result in a total of 16 heatwave days in May.

According to Rantanen, the exceptionally warm temperatures -- around 10C above average for May -- could be an indication that a hot summer was on its way, increasing the risk of forest fires and drought.

"The unusually warm May 2018 turned out to be a precursor for a very hot summer," he said.

The balmy weather was due to a persistent high pressure system situated over Northern Europe for several weeks, in combination with climate change, according to Rantanen.

"We can say for certain that this heatwave is about two degrees warmer in the current climate than it would have been in a pre-industrial climate", he said, warning that extreme weather events such as heatwaves across the globe "would only get worse as long as we are emitting greenhouse gases into the atmosphere".

Earlier this week, Finnish meteorologists issued a rare May heatwave alert, warning of the health risks connected with hot weather.

Typically, such warnings are issued in June at the earliest.