Amazon has been accused of "wasting millions" on a major advertising campaign calling for customers to visit its warehouses.
In recent weeks, the retail giant has launched a series of TV, print and online promotional pushes inviting customers to "take a look" at the company's operations following years of negative coverage over warehouse conditions.
But the GMB union called the move a stunt and demanded the company use its resources on staff to "stay safe and healthy at the end of their shift".
It comes after recent reports found that an ambulance was called out to a UK Amazon warehouse once every two days last year, with staff reportedly breaking bones, collapsing and suffering serious falls.
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The new TV adverts include workers at the company praising Amazon as an employer before asking viewers to "take a look for themselves" by visiting their local fulfilment centre.
Amazon said that "tens of thousands of people" have now visited its 17 warehouses across the UK but it is keen for more people to witness its inner workings.
The company said the advertising campaign is part of a long-held desire to encourage people to see "what it's really like in an Amazon fulfilment centre".
Bosses said they have welcomed a variety of visitors, including school groups, curious customers and small business owners who sell products through its website, on the tours.
During a recent visit to Amazon's Peterborough site for the 75-minute tour, the PA news agency was accompanied by a dozen people, primarily consisting of retirees keen to inspect the site.
The retailer has been keen to increase transparency of its UK operations after union workers staged protests at the company's sites earlier this year, accusing it of having "unsafe" working conditions.
GMB national officer Mick Rix accused the company of spending millions to improve its image instead of addressing staff safety concerns.
He said: "Our investigations prove Amazon is an incredibly dangerous place to work.
"Hundreds of ambulance callouts, workers suffering electric shocks, heart attacks and even miscarriages.
"So rather than waste millions on flash advertising campaigns, why don't Amazon just get round the table with us so we can work together and make sure their staff stay safe and healthy at the end of their shift?"
Amazon said the company offers visitors a view of a "safe and modern environment" on its tours.
An Amazon spokesman said: "We're proud to show the faces and stories of the employees who fulfil customer orders in homes across the UK.
"But don't take our word for it, come see for yourself - we're proud of our employees and would love for anyone to come tour our sites."
On the reports of safety concerns, Amazon said the accusations are "simply wrong and misleading when attempting to portray Amazon as an unsafe workplace".
The retailer claimed that the reports have "sensationalised" the number of ambulance calls and "include personal health reasons", as well as those directly related to work.
The spokesman added: "Amazon is a safe place to work.
"We benchmark against UK national data, published by the Health & Safety Executive, confirming we have over 40% fewer injuries on average than other transportation and warehousing businesses in the UK."