Roads have begun to melt and fans and air conditioning are placing massive demands on electricity suppliers as forecasters predict an all-time record high temperature for Britain tomorrow, when the mercury could nudge 38C.

The heatwave conditions have led to increased demand for electricity, forcing National Grid to call for power stations to make more power available. National Grid sent out a system warning of insufficient capacity margin, of the sort that is usually only seen in winter.

"We are starting to see much higher summer demand as Britain gets a taste for air conditioning, " a National Grid spokesman said.

The heatwave has coincided with a time when many power stations have been shut down for maintenance and repair.

While weather forecasters had predicted many readings of 33-34C (91-93F), especially in the south east of England today, by 4pm, it was only Wisley, Surrey which reached 33 degrees. The other highs were 32.6 in Heathrow and Bedford.

While final results won't be calculated until after 6pm, it looks likely that today will have been the hottest of the year so far surpassing the previous highs of 32.7 at Heathrow yesterday and prior to that 32.4 on June 12.

The Evening Standard this afternoon carried out makeshift temperature readings on public transport, and reported 52C heat on one London bus and 47C in the Tube.

The temperatures being experienced at the moment are higher than those in Miami, Athens, Ibiza, Crete, Malta, Nice, Tenerife and Saigon.

The Department of Health has issued a level three heat wave alert - the second highest level - this week, which requires primary care trusts and care services to monitor the impact of the heat and provide additional care for those most at risk from adverse effects such as dehydration and heat stroke.

Road gritters today spread crushed rock dust across roads to create non-stick surfaces and curb the melting tar. Meanwhile high concentrations of smog were forecast for the Midlands, the north of England and the south east tomorrow and Thursday.

Met Office spokesman Barry Gromett said that there were some simple procedures people could take to keep their body temperatures down.

"It’s important to keep hydrated with water or juice - things like tea and coffee are no good as they might dehydrate you," he said.

"Around the most intense period of the day, at midday when the sun is at its hottest it is very important to wear a hat and sunscreen."

Mr Gromett added that it was a good idea to have windows open to create airflow through the bedroom at night, or to use a fan.

"One of the most important things is to try and keep the bedroom as cool as possible for the overnight period. Overnight, the body recovers from the heat of the day before it has to suffer the stress of another day."

But the big heat could all end tomorrow night, when thundery storms are forecast for the south west of England, and will spread though the rest of England and Wales on Thursday.

This year has so far been the fourth warmest summer on record, beaten only by 1976, 1995 and 2003.

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