An official heatwave is expected before thunderstorms hit the UK later in the week, forecasters have said.

The Met Office has said to expect the early part of this week to be hot, with temperatures reaching around 30C in London and an official heatwave possible by Tuesday.

However, yellow weather warnings are in place for thunderstorms for much of the UK starting Thursday, which could bring surface water flooding and gusty conditions.

Yellow heat health warnings have also been issued across most of England by the UK Health Security Agency (UKHSA).

All areas of England, except the North East and North West have been included in the warning, which is in place until Wednesday.

The UKHSA warned that expected hot weather may have “significant impacts” on the health and social care sector across the South East and London, with minor impact elsewhere.

Met Office spokesperson Nicola Maxey said plenty of sunshine was expected across England and Wales on Monday, with temperatures hitting mid to high 20s in many areas.

She said: “We could see temperatures reaching around 30C in the London area and South East, while it will be a little bit cooler near the coast.

“Through the evening a weather front starts to push its way in to the far North West, but it weakens as it moves, bringing some cloud and light patches of rain over the highest ground further north in Scotland and north-west England.”

Ms Maxey said Tuesday was expected to reach 30 to 31C with sunny dry days in the South East.

She added: “As we move into the middle of the week we start to see an increasing risk of thunderstorms developing, particularly in parts of England and Wales, and the rain is such that it could cause some local disruption.

“This is the sort of rainfall that could cause surface water flooding and heavy rain that takes a while to clear away.

“There could also be some lightning potential for hail and gusty conditions.”

Most of southern England, the Midlands and much of the north of England are covered by the alert, which comes into force at 12am on Thursday and ends at 11.59pm.

Heatwave thresholds could be met across some areas from tomorrow, but Ms Maxey said a change of weather type was expected with a return to cooler temperatures by next weekend.

The threshold is met when a location records at least three consecutive days with maximum temperatures exceeding a designated value, according to the Met Office.

This is 25C for most of the UK, but rises to 28C in London and its surrounding area, where temperatures are typically higher.

The hottest day of the year so far saw a temperature of 31.9C recorded at St James’s Park in central London on July 19.

London Fire Brigade assistant commissioner for prevention and protection Craig Carter highlighted the risks of drowning when people attempt to cool off from the heat.

He said: “Despite the heat, waterways are still cold enough to put a person into cold water shock which can affect anybody, no matter their fitness.

“It can cause panic, anxiety, disorientation and loss of muscular control, which causes a person to gasp for air – inhaling water as a result.”

Mr Carter advised people experiencing difficulty in the water to float, relax and to try to breathe normally before calling for help.

He also asked people not to take disposable barbecues to parks, as they can cause grass fires when the ground is dry.