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Since the 1980s, RTS meters have used a longwave radio frequency to switch between peak and off peak rates.

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.

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."

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.