The amount of rented accommodation could be seriously reduced as a result of a new law to crack down on anti-social behaviour, it has been warned.
The bill is due to become law in October
Local authorities and letting agents in Scotland say they did not have enough time to object to the changes.
But the MSP who introduced the clauses at a late stage of the bill's passage at Holyrood has defended the changes.
A licensing system for all private landlords will be introduced, along with a register of their property.
This should enable local councils to control landlords - and, through them, their tenants.
However, the Association of Residential Letting Agents believes that rents will rise by at least 5% and that some landlords will be forced to withdraw from the market.
It also warned that the legislation could prompt developers to reconsider building accommodation for rent.
Local authorities are also concerned at the changes as they will have to carry out the registration and licensing processes.
Glasgow City Council said the task would be enormous and largely unworkable.
The Anti-social Behaviour Bill is due to become law in October.
The contentious clauses, dealing with anti-social neighbours, were rushed through during its second and third stages in the Scottish Parliament.
The stage two amendments were brought forward in the name of Cathie Craigie, the Labour MSP for Cumbernauld and Kilsyth.
She said: "I realised that they were substantial amendments, but we have been discussing them in parliament since 1999."
She said the vast majority of landlords in the private rented sector were professional, but some could only be described as "rogues".
Mrs Craigie said that the people who found themselves living next door experienced problems identifying and getting in touch with such landlords.
"It causes problems within neighbourhoods and it causes problems for the local authorities, the police and those who have to deal with complaints.
"The easiest way to ensure that we know who the private landlords are, and that they are fit and proper, is to have a registration scheme."
She said it was a simple scheme which was no more complex - and should be no more expensive - than the method of licensing taxi drivers in cities like Glasgow.
"Registration will be a way to clean up that whole sector, and the good and decent people who are out there working professionally at it will have the tarnish taken away.
"All private landlords appear to be getting tarnished with the same brush."