The study found many see property as a safe, long-term investment

The rapid growth of the private rented sector is the focus of a new study led by University of Ulster.

The study found more small-scale landlords are buying into the market, but warned some could suffer from sudden changes in economic conditions.

Built Environment lecturer Paddy Gray said there was "evidence of increasing numbers of vacant properties".

"By 2004 there were over 12,000 vacant properties, bringing the total number of private rented properties to 75,000," he said.

The research suggests a number of factors have encouraged the view that property is a safe, long-term investment.

Planned reform of the housing benefit system, with housing benefit paid directly to the tenant, is causing concern for many landlords Paddy Gray University of Ulster

These include poor performance of pension plans, availability of buy-to-let mortgages, rising house prices and low interest rates.

But they warned that half of landlords had loan-to-value ratios - the amount of money a bank or building society will lend you as a percentage of a property's value - of 76% or more.

The researchers said this could leave them vulnerable to interest rate increases or periods of vacancy, which might mean an unsustainable investment.

"Planned reform of the housing benefit system, with housing benefit paid directly to the tenant, is causing concern for many landlords, and increases uncertainty in the sector," Mr Gray said.

"There is a fear that these changes could lead to landlords with housing benefit tenants exiting the sector or letting to non-housing benefit tenants.

"This scenario may have serious consequences for the Northern Ireland Housing Executive, as these tenants would be forced to seek accommodation in the social sector."

He said this risk should not be exaggerated as most landlords also indicated that they would not sell-up even if they were making a loss.

Mr Gray's research colleagues were Professor Paddy Hillyard of Queen's University and Ursula McAnulty of the University of Ulster.