Forecasters said the weather in the first half of this month was warmer on average than June, July and August.
There has been a mean temperature nationally of 15.9C - 3.3C more than the September average.
The unexpected weather has been explained by warm and humid air being drawn up from the continent.
The hottest temperature of the year, 34.4C, was recorded in Gravesend on September 13.
Despite this the Met Office also said there had been wet conditions across the UK, with an average of 46.8mm of rain falling and many areas experiencing severe thunderstorms.
A return to more ‘usual’ September weather is expected over the next week though, with temperatures reducing and frontal systems coming in from the Atlantic.
But there is set to be a north west/south east split through the second half of the month - with the south of the UK seeing some of the brighter weather.