Temperatures on Wednesday are expected to equal the hottest day of the year so far, as a heatwave spreads across the UK amid warnings of thunderstorms.

Daytime temperatures are forecast to reach highs of 30-32C in parts of central and southern England and eastern Wales.

The Met Office said “many more locations” would meet its heatwave criteria following Wednesday’s warm weather.

However, some areas could see as much as a month’s rainfall in a few hours on Thursday which could cause flooding, the weather service warned.

A yellow thunderstorm warning is in place for London, Essex and much of the South East on Wednesday from midday until 11.59pm.

The weather service has also issued a wider yellow thunderstorm warning covering much of England and Wales except the far South West and North West on Thursday from 1am until 11.59pm.

Met Office deputy chief forecaster Dan Holley said the most intense impacts on Thursday were “likely to be focused on central, southern and South East areas of England”.

He added: “Here some locations will witness torrential downpours, large hail and frequent lightning.

“A few places could see 50-100mm of rainfall in a few hours.”

The Met Office said it was “likely” that some weather warnings may need to be issued at short notice ahead of the development of the most intense thunderstorm activity.

Tuesday was the warmest day of the year so far with a temperature of 32C recorded at Kew Gardens and Heathrow.

Met Office spokesman Oli Claydon said the hottest temperatures for Wednesday were likely to “shift further west” to regions such as the West Midlands and parts of the West Country.

Mr Claydon told the PA news agency: “We’re expecting to see many more locations with their temperatures today taking them over the heatwave criteria – with three days over their threshold temperature.

“We’re expecting it to be fairly widespread across parts of the south of England, particularly the South East.”

There was a large contrast in temperatures overnight with a minimum of 19.9C recorded on the Isle of Portland in Dorset and a low of 1.7C at Tulloch Bridge in Scotland.

Mr Claydon said it was a “pretty uncomfortable night for sleeping” in the South with “high temperatures, but also high humidity”.

He added: “It was very nearly what we call a tropical night where temperatures don’t get below 20C.”

The Met Office spokesman said a weather front approaching from the northwest on Friday would bring “fresher air and the end of the heat” – with lower temperatures expected by the weekend.

The London Fire Brigade urged people to be careful while swimming as the hot weather continues.

Assistant commissioner for prevention and protection Craig Carter said the risk of drowning increased when schools break for summer.

Mr Carter added: “Often, young people are encouraged by their peers and aren’t aware of the dangers – no matter how experienced a swimmer you are, it can be impossible to know how deep water is or how fast it is moving.

“Even on hot days, the water temperature can be cold enough to bring on cold water shock, causing panic, anxiety, disorientation and loss of muscular control. This can cause you to gasp for air and inhale water.”

Charities have also issued warnings as the heatwave spreads, with Shelter advising it could pose a “big risk for people who are homeless on the streets”.

Dogs Trust also warned there was “no safe amount of time to leave dogs alone in a car” during the hot weather.