Dan Channer, managing director of Oxford firm Finders Keepers, said Labour’s proposals would “inevitably cause rents to increase and agency standards to decline”.

He hit out at Labour’s package which includes fixed three-year tenancies, strict limitations on how landlords could give notice to tenants, rent capping, and the banning of fees charged to tenants.

Channer said that three-year tenancies would need to be executed by deed, and that longer tenancies could create more contentious relationships.

He also said that making it illegal to charge fees would mean that agents’ would have to reduce their levels of service.

Channer said: “Labour has failed to address the real problems in the sector, while trying to create new ones.”

Knight Frank – which simultaneously reported an annual fall of 4.3% in “prime rents” in the home counties and a 1.6% annual fall in prime central London – said Labour’s proposals could be “particularly counter-productive” in the capital.

Tom Bill, a researcher at the firm, said: “Rental markets vary drastically across the capital so any one-size-fits-all approach to capping increases could potentially deter investment in the UK market that needs it most.

“Landlords need a certain level of return so capping rents while sale prices float freely will dissuade many investors and squeeze rental stock levels. The result could well be upwards pressure on rents.”

Tim Hyatt, head of lettings at Knight Frank, said: “The vast majority of buy-to-let landlords with mortgages will have a restrictive covenant in their mortgage deed prohibiting a tenancy term of greater than 12 months, meaning there will be a need for significant industry change in this regard.

“There is also a question over the switch of fees from tenants to landlords that could result in increasing rental levels or reducing appetite amongst investors to remain in the sector.”