Landlords are taking their flats off the open market, and advertising them instead on holiday websites like Airbnb, warns an industry report.

The Residential Landlords Association (RLA) claims the trend will mean fewer homes for rent, exacerbating the housing shortage.

It claims that rents could rise as a result, particularly in London.

However, Airbnb disputes the findings and denies that a growing number of its clients are landlords.

In its report, the RLA says 41% of properties advertised on Airbnb in the capital are multiple listings - in other words involve one owner advertising several properties.

There are particular tax advantages for landlords who rent out their properties as short-term holiday lets, rather than rent them out with long tenancy agreements.

By next year, such landlords will not be able to claim tax relief on mortgage interest payments.

In other cases, the RLA is concerned that landlords using Airbnb to rent out properties for more than three months may be doing so to avoid planning regulations.

"Given the pressures faced in the capital it is important that properties advertised as being available for more than 90 days a year are genuine holiday lets with appropriate planning permission," said RLA policy director David Smith.

"Otherwise, as well as taking rental stock off the market for those looking for somewhere to live, they are also putting tenants in a vulnerable position without all the protections offered by a tenancy agreement."

In other countries Airbnb is encountering tougher regulation.

Authorities in Berlin have passed laws which limit the likes of Airbnb to renting out rooms, rather than whole flats or houses.

Since doing so, rents have reportedly moderated.

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'Misleading research'

However Airbnb said there was no evidence that those letting out properties for more than three months did not have planning permission.

And while some properties may be "available" for longer periods, they are often actually rented out for shorter periods.

"This is misleading research that deliberately confuses availability with nights booked," said an Airbnb spokesperson.

"Rather than removing housing stock, hosting on Airbnb puts money in the pockets of regular Londoners and helps them afford living costs in one of the world's most expensive cities."

The RLA research suggests that as many as 21,861 London properties are now being advertised on Airbnb, a figure that rose by a quarter between February and June this year.

It says that number is evidence of increasing "commercialisation" of the website.

The RLA is also concerned that many tenants may be sub-letting flats to short-term visitors.