British households have been told to prepare for blackouts by keeping torches and warm clothes handy as the coronavirus lockdown continues.

Energy firms have suspended all non-essential work as they brace for a potential shortage in engineers caused by staff sickness and self-isolation.

The National Grid has sought to reassure the British public it can cope with the surge in demand as people stay indoors and work from home during the outbreak.

However, UK Power Networks, which provides electricity to the southeast and east of England, including London, has written to vulnerable customers with advice on what to do if there is a power cut.

In a letter to those on the firm’s priority services register, seen by The Daily Telegraph, UK Power Networks gives customers guidance on how to stay warm, keep medicines and food as cold as possible and how to make sure they can call for help.

Customers are advised to keep a “torch handy” as well as “a hat, gloves and blanket” and to trap heat inside their property by closing curtains and doors to any unused rooms.

Vulnerable customers, including those who are elderly, have a disability or medical condition, or have children under the age of five, are told they should keep a power bank fully charged in case their mobile phone battery dies and to use a corded telephone if they can.

The electricity firm also advises fridge and freezer doors should be kept closed to keep essential food and medicines cold.

Food should keep for between four to six hours in the fridge and 15 to 24 hours in the freezer if you can avoid opening it, it added.

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In an update on its website, UK Power Networks said it was “postponing lower priority work” but that all essential work would continue “including fixing power cuts on the rare occasions they happen”.

In the event an engineer needs to enter a home to restore power, they will wear protective equipment, stay in a different room from people who are infected or self-isolating, and avoid touching surfaces, the power distributor said.

It added: “Put simply, our top two priorities are the safety of our staff and customers and keeping the lights on, so that essential services are provided to our communities and that today’s high-tech lifestyles can continue.”

Nicola Shaw, UK executive director for National Grid, has insisted no one should be concerned about their energy supply as more of us stay at home.

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She added: “In fact, demand across the country is expected to reduce; largely owing to a decrease in energy use from industrial consumers, which is likely to be greater than the increase in domestic demand as people stay at home.”

Other energy firms, including Electricity North West and Western Power Distribution, have reassured customers they have “extensive and robust” plans to minimise impact on them.

While ScottishPower Energy Networks said it had “special measures” in place to protect power supply for “critical national infrastructure and public service sites; including hospitals, nursing homes, food supply chain businesses, Ministry of Defence sites and prisons”.

The Energy Networks Association (ENA) reassured customers the network is operating as it should.