A UK union has warned Amazon's Prime Day sale could be a "hive of infection" after eight people tested positive for Covid-19 at a warehouse in Coventry.
The GMB union said the retailer had cut corners on social distancing and the site was "crammed" with workers.
Prime Day, which runs on Tuesday and Wednesday, offers discounts to Prime subscribers and is one of Amazon's busiest events.
Amazon called the union's claims "scaremongering and irresponsible".
It came as campaign groups urged consumers to boycott Prime Day over Amazon's business practices and workers in Germany went on strike.
All of the workers who tested positive in Coventry had been identified through Amazon's in-house testing facility and were asymptomatic, according to the retailer.
But the GMB Union said results took at least a week to come back, and "terrified" workers were told they would not be paid if they did not come in to work in the meantime.
Senior GMB organiser Amanda Gearing said: "Amazon's recklessness could turn Prime Day into a hive of infection.
"Managers are cutting corners and ignoring social distancing rules to speed up processes to achieve targets.
"If Amazon doesn't want to be responsible for the further spread of this deadly virus, it needs to stop flooding facilities with agency workers to maximise profits, enforce social distancing and send anyone home on full pay who may be infected until either the 14 days is over or they test negative."
Amazon rejected the criticisms saying safety was always "front of mind".
A spokesman added: "We've already spent more than $800m on Covid-19 safety measures, with investments in personal protective equipment, enhanced cleaning of our facilities, new process paths that allow for effective social distancing, and developing our own Covid-19 testing capabilities.
"These are the actions of a company taking responsibility for the safety of its employees.."
It came as UK campaign group Ethical Consumer urged shoppers to avoid Prime Day over Amazon's social and environmental failings.
The sale is a big earner for Amazon, exceeding Black Friday and Cyber Monday, with shoppers buying 175 million items during the event last year.
But Ethical Consumer said the online retailer did not pay enough tax in the UK; had a poor track record on environmental issues such as climate change and conflict minerals; and had been criticised over its approach to workers' rights in its supply chains.
"At a time when we need to respond to the Covid and the climate crisis we are asking all consumers to use their spending power to 'build back better' by supporting businesses that have an environmental or social focus," said Tim Hunt, director at Ethical Consumer.
The British Independent Retailers Association also urged shoppers avoid Prime Day and support struggling independent retailers instead, saying many were under strain due to the pandemic.
Chief executive Andrew Goodacre said: "Despite the lure of the convenience of the internet, nothing can beat the positive experience of buying from a local independent retailer; knowing that money spent in a local shop will, in turn, be spent in the local economy."
Amazon said Prime Day was its "biggest small business promotion ever".
It also said it made a total tax contribution of £1.1bn during 2019 - £293m in direct taxes and £854m in indirect taxes.
A spokesman added: "The UK has now become one of Amazon's largest global hubs for talent and this year we announced plans to create 10,000 new jobs in the country by the end of 2020, taking our total workforce to over 40,000."
In Germany, a union has also urged workers at seven warehouses to walk out during Prime Day over a long running dispute over pay and conditions.
It also complained Amazon had scrapped a special coronavirus bonus for German staff in May, despite the continuing pandemic.
Amazon said most workers had turned up as usual, and that it offered "excellent salaries" and benefits and working conditions comparable with other big employers in Germany.
Germany is Amazon's second most important market after the US.