Getty ImagesHundreds of thousands of people are at risk of losing hot water or heating when their old type of electricity meter goes out of action.
Energy companies have said it will be "very, very difficult" to replace all Radio Teleswitching System (RTS) meters with smart meters before the old technology is switched off on 30 June.
Campaigners estimate more than 300,000 homes could lose heating - or have it stuck on constantly - in what energy regulator Ofgem has called "an urgent consumer welfare issue".
The government said the industry had to "work urgently to continue to increase the pace of replacements".
Since the 1980s, RTS meters have used a longwave radio frequency to switch between peak and off peak rates.
The technology is becoming obsolete and energy companies have a deadline to change their customers' meters by 30 June.
At the end of March, there were still 430,000 households using RTS meters for their heating and hot water, according to Energy UK, which represents energy companies.
It said more than 1,000 RTS meters were now being replaced each day.
But based on the 430,000 figure, this daily rate would need to be more like 5,000 to stand a chance of reaching everyone.
Ned Hammond, Energy UK's deputy director for customers, told BBC Radio 4's You and Yours the rate of replacement was rising, but added: "Obviously we'd need to increase from there significantly still to replace all the meters by the end of June."
Asked whether it was impossible to get every RTS meter switched over by 30 June, he said: "I wouldn't want to say impossible - but clearly very, very difficult to get to that point."
Simon Francis, from campaign group the End Fuel Poverty Coalition, said the Energy UK figures suggested more than 300,000 households could be left with a meter that doesn't work from 1 July.
He added: "With pressures on the replacement programme growing and with limited engineer availability, especially in rural areas, there's a real risk of prolonged disruption, particularly for vulnerable households."
RTS meters typically control heating and hot water on a separate circuit to the rest of the household's electricity, so things like plug sockets and lights are unlikely to be affected by the switch-off, Ofgem said.
The RTS network was originally planned to be switched off in March 2024, but this was extended to give energy companies more time to get through everyone.
Energy companies are still targeting 30 June "as things stand", Mr Hammond added, and are developing plans for a "managed and very careful phase down of the system", aiming to protect vulnerable customers.
One challenge of changing everyone on to the new system is a distrust of smart meters. The BBC has previously found that smart meters can sometimes give inaccurate readings and can work worse or better depending on where you live.
Jane from Norfolk told the BBC she is on an RTS meter and does not want a smart meter but feels as if she is being forced into getting one. She is currently on an Economy 7 tariff and does not want to switch.
"It's not yet lawful to say I've got to have one. And I really, really don't want one. I'm perfectly happy with the way things are," she said.
Diane Gray, who lives near Cockermouth in Cumbria, uses RTS to control the heating and hot water in her home on an Economy 7 tariff. She wants a smart meter but has been told one won't work in her house.
In December, her supplier wrote to her to say: "At the moment we're not able to install a new meter in your home that works with your current meter's heating set up. Please bear with us. We are working hard on a solution for your meter type."
She's since received another notification that a smart meter will be fitted in early June.
"I've got no idea where it's going to leave us," she told the BBC.
"It is very concerning. Because they're doing it in the summer, come the winter I keep thinking there must be some solution they're going to give us before we need to start using the heating."
If your energy supplier cannot fit a smart meter in your home, Ofgem says your supplier must install a "suitable meter" with no disruption to your service.
Getty ImagesAccording to Ofgem, you may have an RTS meter if
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