Power companies are being asked to generate more electricity following a surge in demand as consumers switch on air conditioning systems in a bid to beat the heatwave.
The National Grid yesterday said it had issued a Notice of Insufficient System Margin (NISM) to power firms, signalling that additional reserve was needed between the hours of 10am and 1pm.
Last week, the average cost of the wholesale market trebled after a notice was issued due to demand from sweltering customers and a number of power stations being closed for repair work.
The average cost of wholesale electricity tripled in the wake of that warning.
A spokeswoman said yesterday: "A number of power stations were down yesterday and demand remains high as consumers are using air conditioning and refrigeration due to the heat.
"There is no need to panic as domestic customers will not be affected so we are not issuing an alert.
"We are asking the power companies to reduce demand or generate more electricity. This is a standard tool of the network."
NPower fired up two extra oil-fuelled power stations at Fawley in Southampton and Littlebrook in Essex last week, while Centrica increased generation at its gas-fired station at South Humber in Lincolnshire.
Even after the NISM was removed, wholesale prices remained high after hitting an average of £260 per megawatt hour last Tuesday, compared with £81 on Monday. The figure peaked at £330 per megawatt hour on Tuesday.
The notice reflects the fact that the country's safety net of spare generation capacity drops below a certain level.
Meanwhile, the sweltering weather has been bad news for retailers, figures showed yesterday.
Footfall in stores dropped by five per cent last week compared with last year as soaring temperatures kept shoppers away.
Department stores fared worse, with customer numbers down 5.5 per cent.