The EPC system is subjective. Craig Powell*, an energy assessor, estimated that one in four EPC ratings was wrong. “Two guys can do the same property and come up with different figures,” he said.

Julia Rennie, 59, spent £4,000 to replace old night storage heaters in two of her flats with electric radiators.

Her tenants’ energy bills dropped dramatically. But the EPC assessment did not take into account that they previously had to plug in fan heaters in the evenings at peak rates. The EPC ratings dropped, and each flat had a different result: in one, the EPC fell from C to D, in the second it fell from C to E. “The flats are virtually identical, it doesn’t make sense,” said Ms Rennie.

In September 2020, the Government published plans to make EPCs more reliable and is reviewing the changes needed to encourage people to improve the energy performance of their homes. But experts are frustrated with the lack of progress so far. “They have done multiple consultations, but nothing has actually been done,” said Ms Ralston.

Mr Powell also warned that government grant schemes, such as the renewable heat incentive, created opportunities for fraud. He said having a poor EPC rating would financially benefit installers and he had seen evidence of manipulation.

“It was clear the EPC they used to claim the grant was deliberately inaccurate,” he said. “There’s a lot of nudge nudge, wink wink, don’t look for certain things.”

A Government spokesman said: “No properties will be made unmortgageable by our plans to boost energy efficiency in homes.

“Our reforms will deliver a fairer system for all, supporting homeowners and landlords to improve their home energy performance, cut energy bills and increase consumer choice.”