THE Met Office has issued a yellow weather warning for the south west, including Wiltshire, as isolated heavy, and possibly severe, thunder storms are expected to develop on Friday evening.

These are likely to become more frequent later and spread northwards.

Some torrential downpours are possible leading to localised surface water flooding, with large hail and frequent lightning also possible hazards.

The warning, issued today, is valid from 7pm on Friday until noon on Saturday.

The chief forecaster said: "The public should be aware that there is a chance of some very localised significant disruption.

"On Friday evening another hot and increasingly humid airmass is expected to spread into the south from the continent.

"This airmass looks conducive to the development of isolated thunderstorms during the evening across England and Wales which could become more frequent later in the night as they spread northwards towards Scotland.

"With large amounts of energy again available in this atmosphere storms could be severe, with torrential downpours and large hail - 30mm of rain is possible in less than an hour, with as much as 50mm possible in three hours very locally.

"Large uncertainty remains regarding areas most at risk from storms, and many areas will miss these altogether, staying largely dry. However, where they do develop, disruption is possible."

MeteoGroup's Nick Prebble said: "Temperatures will be suppressed today, about 10C lower than yesterday with highs of 25C or 26C. It will be noticeably cooler but still warm.

"Friday will likely stay nice and dry across most places and temperatures will be a little warmer, around 27C or 28C, although there will be some unsettled weather Friday night with more thunder storms."

He added: "Looking into the weekend, it could top 30C again on Saturday in the South East and London and temperatures will be above average elsewhere too, going in to Sunday."

Moderate levels of air pollution are expected to remain in the southern and eastern areas of England throughout today and Friday before subsiding over the weekend, according to the Department for Environment, Food and Rural Affairs (Defra)

Earlier it had warned those vulnerable to the effects of pollution to be cautious as the high temperatures caused pollution levels to spike.