The Met Office has so far recorded highs of 25.7C (78F) occurring at Valley near Holyhead in Wales.
Meteorologist Becky Mitchell said: “There is plenty of sunshine out there today, it’s a warm day. There’s barely any clouds across most of the UK.
“Today we’re expecting temperatures around 26 degrees, maybe 27, which would be above average for the time of year.
“Obviously, at this time in September it averages 17 or 18 in the south and about 14 in the north.”
However, this unusually warm weather is set to come to an abrupt end on Saturday night as the remains of hurricane Humberto hit the western side of Britain.
Ms Mitchell said that the last of the category three storm has left Bermuda and is travelling across the Atlantic towards the UK.
Much of the tropical weather system has dissipated but it is still likely to bring an area of low pressure that will see an end to the current pleasant conditions.
She said: “There is a lot set to change tonight. We have got some heavy rainfall coming, mainly in the west. It will come in from Northern Ireland, Wales and western parts of England.
“The rain is set to come in tonight and it will be quite cloudy and wet from tonight into tomorrow, where we will start to see that heavy rain arrive.”
She added: “We have got a low pressure system coming from the Atlantic from tomorrow that will drive the rain in from the west.
“Into next week it’s going to be quite unsettled. Some of these weather systems are the remnants of hurricane Humberto.”
The meteorologist said that the warm air from the continent that caused Saturday’s sunny weather is likely cause more rain as these two weather systems collide.
“It means it’s quite a moist atmosphere, which often brings some heavier rain. The outlook into the start of October does look to stay largely changeable.”
The warmest September day on record was 35.6C (96F), which was recorded at Bawtry in South Yorkshire on September 2 1906.
The highest recorded temperature last September was 26.5C (80F) at Cambridge Botanic Gardens.