Homes could be hit by rolling three-hour blackouts this winter if the UK fails to import enough energy, according to National Grid.
Households are being encouraged to scale back on how much electricity they use in a bid to avoid power cuts.
The warning comes despite Liz Truss insisting there would be no need for blackouts during the Tory leadership contest.
Asked by the LBC broadcaster Nick Ferrari at the final hustings event if she could rule out energy rationing, she replied: “I do rule that out. Yes.”
Planned blackouts have not been seen in Britain since the 1970s.
Such a move would need to be approved by King Charles on the recommendation of the business secretary, Jacob Rees-Mogg.
However, National Grid insisted that scenario remained “unlikely” although it cannot be ruled out entirely.
“In the unlikely event we were in this situation, it would mean that some customers could be without power for pre-defined periods during a day – generally this is assumed to be for three-hour blocks.”
However, National Grid hopes that by paying people to charge their electric cars at off-peak times and firing up backup coal plants it can offset the risk of blackouts.
Jake Rigg, National Grid's director of public affairs, said: “The demand flexibility service is a first of its kind and a smart way for signed-up consumers in homes and businesses to save money and back Britain.
“If you put your washing machine or other electrical appliances on at night instead of the peak in the early evening, you can get some money back when we all need it.
“The service is due to launch in November, so watch out for further details soon. This really is a window into the future where a flexible energy system will be cleaner and lower cost to alternatives.”
Energy regulator Ofgem said: “We have one of the most reliable energy systems in the world and we are in a favourable position.
“However, it is incumbent on a responsible and prudent energy sector to ensure the right contingency measures are in place.”
A government spokesperson said: “The UK has a secure and diverse energy system.
“To strengthen this position further, we have put plans in place to secure supply and National Grid, working alongside energy suppliers and Ofgem, will launch a voluntary service to reward users who reduce demand at peak times.”