The heatwave is forecast to last into the weekend and will likely peak on Saturday in London. Temperatures of at least 30C are forecast for Thursday, Friday and Saturday.
It has not been this hot since June, and while it is unusual for September to see temperatures exceed 30C, it is not unheard of.
Economic experts said the September heat would help boost spending after a soggy summer that depressed sales on the high street.
September’s hottest day ever recorded was in 1906 when a temperature of 35.6C was recorded in South Yorkshire.
Climate change is bringing hotter and longer heatwaves with meteorologists projecting that by 2070, 30C on two or more days will become more likely, with southern parts of the UK experiencing this 16 times more frequently than today.
US-based researchers Climate Central have calculated that this month’s late heatwave was made five times more likely because of climate change.
The UK Health Security Agency has issued an amber warning for heat lasting until Sunday at 9pm, meaning extra stress will be placed on the health service because of the weather, with a greater risk to those over 65 or with respiratory and cardiovascular diseases.
There is also a chance of tropical nights in southern parts of the UK, which are when the temperature does not dip below 20C overnight.
Councillor David Fothergill, chairman of the Local Government Association’s Community Wellbeing Board, has warned people to take care in the heat, especially older people, those with children and those who spend a lot of time outside.
He said: “No one is immune to the power of the sun and we know that there is growing evidence to suggest that extreme heat has an adverse effect on our health.
“It may sound like common sense, but drinking plenty of water, keeping our homes cool, avoiding direct sunlight during the hottest hours of the day and using sunscreen are sensible precautions we all need to remember.”
He also said there may be some changes to council services such as waste collection being earlier in the day and extra support for those receiving adult social care.
Around half of councils have also prepared to treat roads to stop them from melting, Mr Fothergill added.