A DIY store exposed hundreds of thousands of customers and staff to a risk of contracting Legionnaires' disease, it has emerged.
A branch of B&Q had a working spa pool on display in April 2005 which contained water contaminated with the potentially deadly bacteria, a council said.
The firm has now been fined £20,000 by magistrates in South Gloucestershire after it admitted breaching the Health and Safety at Work Act by exposing employees and shoppers to the risk of Legionella pneumophila.
North Avon magistrates fined the DIY giant the maximum amount for breaching the regulations and ordered it to pay the council's costs of £8,405.20.
A B&Q spokeswoman said: "The breach occurred when the Longwell Green store failed to follow company process by filling a display hot tub with water.
"This contravenes company policy. When B&Q was made aware of the situation all stores were re-issued with a mandatory notice banning the display of working hot tubs.
"B&Q would like to assure its customers that the health and safety of its customers and employees is of the utmost importance. We are not aware of any sickness reported by customers or employees as a result of this breach."
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David Urpeth from law firm Irwin Mitchell said: "The risks of Legionnaires Disease are well known and it is therefore of huge concern that B&Q exposed thousands of people (both workers and customers) to the risk of Legionella pneumophila.
"B&Q failed in its duty to protect workers and members of the public from exposure to this potentially fatal bacteria.
"I represent many people who have been injured or killed in workplace accidents involving workers and/or members of the public."