Meera Chindooroy of the National Residential Landlords Association, a trade body, said utility bills were landlords’ second-largest cost on their properties after upkeep and maintenance, typically accounting for an eighth of expenditure. Many buy-to-let investors are calling the NRLA’s helpline because they are worried about rising bills, she added.

Kevin Sands*, 62, is a landlord whose portfolio includes nine HMO properties in Essex. In one HMO, the annual gas and electricity bill has jumped by £365 to £2,203, an increase of 20pc.

“The gross rent on a five or six-­bedroom house is about £30,000 per year,” said Mr Sands. “If my costs go up by even 10pc, that is about a one percentage point hit on my bottom line. Instead of a 7pc yield, that’s down to 6pc,” Mr Sands said.

HMO landlords are being squeezed on multiple fronts. Rising interest rates have been a further blow to profit margins.

“Previously, rising bills could be offset by low interest rates. Over the past few years, when we used to refinance a property, the bills often went down by maybe £500 a year,” said Mr Sands.

The Bank of England’s decision to raise Bank Rate from 0.1pc to 0.25pc before Christmas, however, has done the reverse. When Mr Sands remortgaged one of his HMOs at the end of last year, the higher rates meant his costs increased by £100 a month.