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."

Manchester City Council used a crane to remove the 44ft (13.5m) trailer, which had sunk into the road surface in South Street, Openshaw.
The cost of removal and disposal was almost Â£10,000.
Councillor Nigel Murphy said the people who dumped it had "stolen from Manchester taxpayers".
The trailer, which weighed double the legal limit, had been stripped of usable parts and scrap metal, as well as all its identifying features, such as registration plates.
Mr Murphy said investigators suspected the waste had "come from a site used to sort skips".
A council spokesman said it was hoped "someone working in logistics" would recognise the vehicle, which was described as "an old tri-axle steel waste hydraulic ejector trailer".

The 30-year-old, who has scored five times in 16 international appearances, has struggled with injury this year but marked her return to action with two goals against Doncaster last month.
Liverpool captain Gemma Bonner, 25, has also been included.
Injured former captain Casey Stoney - who has 129 caps for England - and Fran Kirby have been left out of the squad.
The Lionesses will face the Euro 2017 hosts in Tilburg at 18:00 GMT in their final friendly of 2016.
England manager Mark Sampson said: "It's a unique chance to play at one of the stadiums where we will play in July and now that we know who we face at the Euros, the intensity and focus in our group will sharpen.
"We are nine games unbeaten and will be fighting to make that 10, but we are expecting a tough game against a fellow contender for the Euros."
The Euros begin on 16 July with Sampson's side opening their campaign against Scotland on 19 July before taking on Spain and Portugal in Group D.
Goalkeepers: Karen Bardsley (Manchester City), Siobhan Chamberlain (Liverpool), Mary Earps (Reading)
Defenders: Laura Bassett (Notts County), Gemma Bonner (Liverpool), Lucy Bronze (Manchester City), Gilly Flaherty (Chelsea), Steph Houghton (Manchester City), Jo Potter (Notts County), Alex Scott (Arsenal), Demi Stokes (Manchester City)
Midfielders: Isobel Christiansen (Manchester City), Jade Moore (Notts County), Jordan Nobbs (Arsenal), Jill Scott (Manchester City), Fara Williams (Arsenal)
Forwards: Karen Carney (Chelsea), Danielle Carter (Arsenal), Rachel Daly (Houston Dash), Gemma Davison (Chelsea), Toni Duggan (Manchester City), Nikita Parris (Manchester City), Jodie Taylor (Arsenal)
Rules on displaying hot tubs in shops and public places should be reviewed, according to a report into a fatal outbreak of Legionnaires' disease.
[NEXT_CONCEPT]
A trailer filled with 90 tonnes of domestic waste was dumped on a street in an act of "fly-tipping on an industrial scale", a council has said.
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Arsenal striker Jodie Taylor has been named in England's 23-woman squad to play the Netherlands on 29 November.