Campaigners warn residents have faced months of uncertainty and misery since the collapse of Britain’s largest care home owner earlier this year.
But plans to hand over some of the homes to Four Seasons Healthcare came under blistering attack at the Labour party conference in Liverpool.
The company was accused of being “cowboys” who would plunge the home into even greater uncertainty.
Four Seasons Healthcare has already come under fire in Scotland after it controversially announced it would close a home in Dundee in the wake of a critical report.
The Rosebank care home houses up to 32 adults with learning difficulties and will shut in March.
In recent days Dundee City Council has warned it could face costs of around £500,000 a year because of the decision to close the home, where inspectors found serious fire risks and unpleasant smells in many of the bedrooms.
Sharon Holder, GMB national officer, asked the conference to back a call from the union that regulators block the transfer of former Southern Cross care homes until it is able to establish Four Seasons Healthcare has financial stability.
She praised some of the 33 operators that are taking over Southern Cross homes.
But she said: “Then there are the Four Seasons of this world; not a cavalry coming over the hill, more like a bunch of cowboys looking to fill their sandbags with more public money before they ride off into the sunset.”
Thousands of residents live in almost 100 Southern Cross premises in Scotland.
There have been fears many may have to leave their homes in the wake of the collapse.
Southern Cross was criticised by labour leader Ed Miliband in a speech earlier this week in which he attacked so-called bad businesses in society.
Last night Jackie Baillie, Scottish Labour’s shadow health spokesman, called for any hand-over to be delayed. Ms Baillie said Four Seasons had a reputation for consistently poor reports from the Care Inspectorate.
“The GMB is right to raise the question of Four Seasons financial position and until the company can provide satisfactory guarantees I think that here in Scotland the Care Inspectorate should also suggest postponing any takeovers,” she added.
Former Southern Cross homes are being divided among dozens of smaller firms, including Four Seasons Healthcare, which branded the GMB claims “irresponsible scaremongering”.
The firm is thought to be due to take over 145 homes by the end of October, making it Britain’s largest care home firm.
Last night a spokesman for Four Seasons branded the GMB’s allegations “substantially wrong”.
The union was guilty of “irresponsible scaremongering” which would “cause unnecessary worry to elderly residents and relatives and to employees”, she added.
She said: “Four Seasons is in good financial health. Unlike Southern Cross it is trading profitably.”