The previous highest temperature for the year was set in June at 32.2C.
The heatwave has already broken the record for the most consecutive days with temperatures above 30C in September, with Saharan dust generating vivid sunsets and sunrises in the clear conditions.
Thursday was the fourth day above 30C, beating the record of three days seen on four previous occasions, most recently in 2016.
Met Office meteorologist Simon Partridge said: “If we do see 30C all the way through until Sunday, which it looks fairly sure it will be, that will be six days in a row that we have reached 30C.
“Previously in September we’ve only reached 30C three days in a row.
“So although it’s not the the hottest spell of weather we’ve had in September, in terms of prolonged hot weather it is twice as long as we have previously had.”
September’s highest daily temperature reading was 35.6C recorded on September 2 1906 in South Yorkshire, according to the Met Office.
But Mr Partridge told PA there was “no chance” that the September daily temperature record could be broken in the coming days.
He said: “There is potential that we might get a little bit warmer over the weekend, not by a massive amount, but enough to make it the warmest day of the year so far again.
“It is always going to be around 32C, close to 33C at the maximum temperature.”
Prolonged heat above 30C leaves older people and those with respiratory or cardiovascular diseases at greater risk, as the UK Health Security Agency issued an amber warning until 9pm on Sunday in nearly every area of England.
A yellow heat alert is in place until the same time in the north east of England.
The Met Office on Friday issued a yellow weather alert for Saturday, warning of potential thunderstorms which could cause disruption on the roads. It warns of potentially hazardous conditions between 2pm and 9pm, should thunderstorms occur. The alert covers much of England including London and the South-East.
Temperatures are expected to remain high overnight into Saturday.
Mr Partridge said: “The biggest knock-on effect at the moment is those overnight temperatures because in parts of south-west England and Wales (Wednesday) night many places didn’t dip below 19C, which is not easy.
“We will continue to see temperatures in the mid to high teens overnight. A few spots could again not dip below 20C, so it’s very warm and muggy nights.”