The UK Met Office has raised the nations' fire severity index to exceptional - the highest level - for much of southern England, and stretching as far west as Abergavenny in Wales.

Met Office meteorologist Marco Petagna told the PA news agency there was a very real risk of wildfires.

"The risk is very high across much of central, southern and eastern England," he said.

"Going into Friday and the weekend it starts to increase further, going into the highest category of exceptional risk."

Mark Hardingham, the chairman of the National Fire Chiefs Council told The Telegraph newspaper he "can't remember a summer like this" in his 32-year career in the fire service.

"We're not going to see temperatures as hot as we saw three weeks ago," he said.

"But that doesn't matter because the ground couldn't get any drier than it already is."

The Met Office has issued an amber heat warning running between Thursday and Sunday, with temperatures set to peak at 36C across southern England and eastern Wales.

Petagna said temperatures would "widely" be in the 30s across the UK, peaking locally in the mid-30s on Saturday.

"The hottest day will probably be Saturday, where we could see 36C in one or two spots," he said.

There is also a heat health alert in place from the UK Health Security Agency, with experts advising people to look out for those who are older or with existing health conditions, as well as young children.

It comes as the driest first seven months of the year in decades and hot spells have left parts of the UK facing looming drought, prompting hosepipe bans and warnings about the impact on agriculture, rivers and wildlife.

The latest analysis from the UK Centre for Ecology and Hydrology has warned low or even exceptionally low river flows and groundwater levels are likely to continue for the next three months in southern England and Wales.

"There are signs that we could get some rain next week, but details at the moment are uncertain," Petagna said.

"What we really need is a few weeks of light rain to soak into the ground.

"Thunderstorms are more likely to cause some flooding issues because the ground is hard, so the water can't sink in."

Australian Associated Press