Fire bosses have raised serious safety concerns about plans to make it cheaper to run electrical appliances at night as a new trial gets underway.
They warned using dishwashers, tumble dryers and washing machines at night puts families at greater risk of being trapped by a fire while they sleep.
A London Fire Brigade spokesman said: “We understand that a balance needs to be struck between energy use, green issues and fuel costs but in our view you cannot put a price on fire safety.
“While the vast majority [of appliances] are manufactured to a good standard and are perfectly safe, we would not advise leaving them unattended overnight.
“If a fire breaks out during the night, the risk to life is always greater as it's very likely people will be asleep and have less time to react and escape the property.”
The National Fire Chiefs Council said it had not been consulted about plans to introduce peak and off-peak pricing.
A spokesman added: “We are conscious of the need to reduce energy bills and supportive of help companies look to provide consumers.
“But we also urge providers to ensure reduced tariffs are not limited to times when most households would be sleeping.”
A spokesperson for Electrical Safety First said: “Fires that occur at night can present a greater risk to people who may be sleeping and their response to the incident, delayed.
"Where practically possible, we advise consumers to avoid leaving appliances like washing machines, tumble dryers or dishwashers running unattended overnight.
"It seems inevitable that during the current crisis more appliances will be running overnight so we strongly advise that members of the public do all they can to reduce their risk by following a few simple steps.
"It is crucial that homes are fitted with working smoke alarms on every floor and that electrical sockets are not overloaded. Now, more than ever, it is imperative to ensure appliances are registered with the manufacturer so that in the event they are subject to a recall the consumer can be contacted directly.
"It is also important that appliances are kept in good working condition and any filters are regularly cleaned, as per the manufacturer’s instructions
"Cables should be checked regularly for any signs of damage and consumers should take care not to overload extension leads with multiple appliances. These simple checks will help protect consumers during an increasingly challenging period for millions.”
Over a million UK households will be paid to turn on their appliances at quieter hours of the day under a new trial to reduce pressure on the National Grid and help lower household bills during an energy crisis.
It will be followed by an overhaul of smart meters that will allow firms to change their prices every half hour.
From Friday, all 1.4million Octopus Energy customers with a smart meter will be invited to join a scheme that will mean they pay less if they cut their normal electricity consumption at certain two-hour periods during the day.
The pilot scheme hopes to spread out energy usage across peak and off peak times to reduce the pressure on the electricity grid as the UK ditches reliable but dirty fossil fuel plants.
Households could be asked to delay a laundry cycle or a dishwasher load when demand on the power grid typically climbs, such as between 9am and 11am and again between 4.30pm and 6.30pm.
If they meet their targets they could earn up to 35p for every kilowatt-hour of electricity saved.
Those in the trial will be told about their two-hour window by 4pm the day before it is scheduled, so they can choose in advance whether to opt in or not.
Octopus expects about 100,000 homes to sign up for the experiment, which runs until the end of March.

Under it, the supplier will set personalised electricity reduction targets for each customer within certain two-hour windows, typically between 40% and 60% of their average usage during these periods.
If they hit it, Octopus will offer free electricity for the remaining electricity used during the two-hour slot.
The trial would be a welcome opportunity for eligible households amid a bill crisis that will see energy tariffs soar by almost £700 from April under a revised cap.
Greg Jackson, founder of Octopus Energy Group, said: "UK households are currently suffering from huge energy bill increases caused by the global gas crisis. We speak to 30,000 customers a day and know first-hand how hard the situation is for many in this country.
"Octopus Energy has pioneered energy trials that put consumers in the driver’s seat, allowing them to save money when energy is cheaper and greener. We are glad that National Grid ESO have now joined us in our efforts to build a smart grid that puts money back into peoples’ pockets and drives the green energy revolution - and this trial is an incredibly important step to make it a reality."
Isabelle Haigh, head of National Control at ESO, said: "System flexibility is vital to help manage and reduce peak electricity demand and keep Britain's electricity flowing securely.
"This trial will provide valuable insight into how suppliers may be able to utilise domestic flexibility to help reduce stress on the system during high demand, lower balancing costs and deliver consumer benefits."
James Eddison, co-founder of Octopus Energy Group, added: "It's a tremendous opportunity to unlock flexibility at an unprecedented scale."