It follows four consecutive years of growth in listings for lodgers, which now account for around a quarter of room adverts on the website.

SpareRoom says the decline comes despite strong demand for rental property with 2.41 prospective tenants for every room that’s available across the UK.

The website is calling for the government to increase the tax-free allowance offered under the Rent a Room scheme to help boost availability.

It says that since the scheme was last updated in 2016, the UK average room rent has increased by 29% to £749 per month, while the allowance has remained frozen at £7,500 a year – equivalent to £625 per month.

SpareRoom estimates there are around 28 million empty bedrooms across England, Wales and Scotland.

It says freeing up just 5% of these rooms could provide accommodation for around 1.4 million people.

SpareRoom director Matt Hutchinson says: “In the middle of a housing crisis we should be doing everything possible to protect supply in the rental market, whether it comes from landlords or homeowners.

“Lodger landlords and the rooms they offer make up a sizeable wedge of the flatshare market and although renting as a lodger doesn’t suit everyone, the flexibility and often cheaper rents are a draw.

“The last time the Rent a Room scheme was altered, supply rose and remained at healthy levels until the pandemic.

“A booster shot in the arm is what’s needed now.

“We know the administrative burden of having to fill out a tax return does deter people from renting out rooms, so the tax-free threshold needs to reflect rents today.

“But we also want to stop those running holiday lets from using the scheme, so we can divert supply into the rental market.

“We shouldn’t be incentivising short-term holiday stays when there’s such a large supply-demand imbalance in the rental market.”