An amber extreme heat warning has come into force, with temperatures forecast to hit 37C (99F) in some parts of the UK over the next four days.

The Met Office alert for southern and central England and parts of Wales runs from midnight on Thursday until Sunday.

It has also issued its highest alert for fire severity over the weekend, warning there is an "exceptional" risk of blazes spreading in many places.

The heatwave will likely affect health, transport and working conditions.

There is also an increased risk of water safety incidents, the Met Office warned.

While the record-breaking temperatures seen in July, when the mercury topped 40C (104F) for the first time, are not expected this week, the heat in some areas could get close to local or regional records.

"This heatwave, temperatures won't be as high. But the big difference with this current heatwave is that it's much more prolonged," said Matt Taylor, a BBC Weather forecaster. 

"We've already seen temperatures above 30C every day this week - Monday, Tuesday and Wednesday - and that will continue until Sunday."

He said temperatures could reach 35C and peak at 37C in some areas between the Midlands and London

In Wales, Cardiff may see temperatures exceeding 30C.

Scotland and Northern Ireland meanwhile are likely to be cooler with highest temperatures in the mid-20s, the Met Office says.

The Met Office's fire severity index, which assesses how severe a blaze could become, is currently "very high" for most of England and Wales.

This will rise to "exceptional" - the highest level of risk - for parts of England and Wales by the weekend, while a warning for "very high" risk of wildfires across southern and eastern Scotland has been put in place until Monday.

The period between January and June this year was the driest in England in 46 years, prompting warnings about the effects on agriculture, nature and wildlife.

More than three inches of extra rain, in addition to what would normally be expected, is needed to overcome the dry conditions in parts of southern England, but no rain is forecast any time soon.

Analysis by the UK Centre for Ecology and Hydrology found low or even exceptionally low river flows and groundwater levels were likely to continue for the next three months in southern England and Wales.

Water companies are being urged to protect essential supplies into a dry autumn, while three firms have already implemented hosepipe bans.