The Health Protection Agency (HPA) report said a hot tub on show at JTF Warehouse in Stoke-on-Trent was the probable source of an outbreak in 2012.
Richard Griffin, 64, and 79-year old William Hammersley, both died and 19 other people caught the disease.
The HPA said hot tubs posed a health risk if not regularly maintained.
BBC News has not been able to contact JTF Warehouse for a comment following the HPA report.
The report said: "This was a significant outbreak and has raised questions on the acceptability of operating spa pools (also known as whirl pools and hot tubs) in public places where the general population has access.
"Spa Pools are known to pose a risk of Legionella if water systems are not rigorously maintained, properly managed and subject to regular chemical controls.
"Operating spa pools on display in indoor spaces, even if not used for bathing, have been previously shown to be the cause of outbreaks in other countries," the report added.
Lawyers Irwin Mitchell are taking legal action on behalf of the families of two men who died, as well as 16 other people who caught the disease.
They are calling for a public inquiry into the outbreak.
Clive Garner from the firm said: "We hope that now this report has been published those responsible for the store will admit liability for the illness suffered by our clients so that we can help them to access the exact level of care and support they now need.
"We are currently working with medical experts to investigate their longer term condition and prognosis and we hope to conclude their cases as soon as possible so that they can begin to move on with their lives."
Rules on displaying hot tubs in shops and public places should be reviewed, according to a report into a fatal outbreak of Legionnaires' disease.