Tuesday is the joint warmest day of the year so far after a temperature of 31.9C was recorded in central London, according to the Met Office.

Recorded in St James’s Park, it ties with the temperature on July 19 in the same location.

The Met Office has confirmed that Britain is currently experiencing an official heatwave and that it is likely to remain very warm for the next few days, although yellow weather warnings have been issued for thunderstorms on Wednesday and Thursday.

Forecasters also warned some areas could see as much as a month’s rainfall in a few hours on Thursday, which could cause flooding.

Meteorologist at the Met Office, Becky Mitchell, said: “We are technically in a heatwave now because the high temperatures… started yesterday, and we’re in the midst of it now as we expect temperatures to continue to be high in the coming days.”

Heatwave criteria is when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold, which varies by region but is 28C in London.

Earlier on Tuesday, a fire broke out in Orpington with four fire engines and around 25 firefighters called to a field where four hectares of straw were ablaze.

Temperatures will remain at 30C or above into Wednesday and Thursday, though thunderstorms are expected in the south east on Wednesday and across much of England on Thursday.

“The heat and humidity we’ve got across England and Wales is going to spark off some thundery showers on Wednesday and Thursday so we’ve got some yellow weather warnings in force,” said Ms Mitchell.

There is a risk of flooding on Thursday afternoon in some parts of the country, but the thunderstorms will be “quite hit and miss”.

The Met Office said Thursday afternoon could see 50 to 100mm of rain in a few hours in some spots, which is more than a month’s worth of rainfall.

The heat is not set to last into next weekend, however, as temperatures are due to return down to the mid-20s in the south as a band of rain moves through the country on Saturday.

To cope with warm evenings, the Met Office has urged Britons to keep their homes as cool as possible.

Official advice said: “Overnight temperatures are going to be very high so it’s quite hard to cool houses down in that situation because you don’t have the relief overnight and temperatures dropping.

“If you can open your windows at night, try and let some of the cooler air in and keep curtains or blinds shut during the day”.