Workers at Amazon's UK warehouses are being told to work overtime to tackle huge demand due to the coronavirus pandemic, despite government calls to restrict social contact.
The GMB union says that workers across at least four different sites were informed that they had to work "compulsory overtime" from Monday.
National officer Mick Rix said Amazon had put "profit before safety".
Amazon said it was working to ensure it can continue to deliver to customers.
Compulsory overtime means that some employees must work additional hours as requested by an employer - if their contract says so.
'Huge spike in demand'
Amazon employs 27,000 people in the UK and has 17 warehouses.
One worker at Amazon's Dunfermline warehouse in Scotland, who asked not to be named, told the BBC that staff in the "inbound goods" department are having additional hours imposed.
The worker, who thinks they have a compulsory overtime clause in their contract, believes this will be for at least two weeks.
They said that there is extra pressure on the workforce to deal with an influx of goods the company is bringing in due to a spike in demand.
Sought-after items include bleach, handwash, nappies, large boxes of rice and powdered milk.
The worker added that these actions were "very rare" outside of the Christmas trading period or Amazon "Prime week", where the firm offers discounts on goods for subscribers.
The worker said other departments in the Dunfermline warehouse are not saying staff should do more hours, but offering them up to 60 hours of voluntary overtime.
An Amazon spokesperson confirmed that the company had ramped up shifts across the UK.
They said: "As demand continues to increase, we are working to ensure we can continue to deliver to the most-impacted customers while keeping our people safe".
"Many of these customers have no other way to get essential items and we want to be sure that we have the right resources in place to deliver on their needs.
"Starting this week, we'll be prioritising the intake and dispatch of items most needed by our customers right now. These are items such as food, health and personal care products, items needed to work from home, books and toys for children."
Amazon also said that there is an exemptions process in place for employees who cannot work additional hours for personal reasons, such as caring responsibilities.
'No regard' for safety
On Monday, Boris Johnson said people should work from home where possible as part of a range of stringent new measures to stop the spread of coronavirus.
The GMB union's national officer Mick Rix called the overtime reports "extremely concerning", and accused Amazon of "imposing its demands on workers without any regard for their safety".
Mr Rix said he was concerned that if staff are overworked, stress will make them more susceptible to the Covid-19 virus.
Sarah Evans, employment law partner at JMW solicitors, said that if a worker's contract has a clause in it that says there "may be an element of compulsory overtime", then a boss is entitled to use it to make them do additional hours.
She pointed out that under Working Time Regulations in the UK, overtime is limited to a maximum of 48 hours per week, averaged over a 17-week period.
Workers can "opt out" of the maximum weekly limit, however, and some are required to do so as a condition of employment.
Hannah Ford, a partner and employment law expert at Stevens & Bolton, said that these were "unprecedented times, but all employers must operate within the law."
She added: "All employers also owe an implied duty to take reasonable care for the health and safety of its employees... this extends to mental health as well as physical health."
Extra delivery workers
Workers in the US have also been posting on social media about working overtime at Amazon fulfilment centres.
Everybody job getting shut down, except Amazon. We're on overtime.— Pacino (@Pope_Sean_Paul) March 16, 2020
Amazon has said it will hire 100,000 warehouse and delivery workers in the United States to deal with the surge in sales due to pandemic.
The online retail giant also said it would increase pay for its staff in the UK, US and Europe.
Amazon said it would increase hourly wages by $2 in the US, £2 in the UK, and €2 in Europe. The company said it expects the pay rises expected to cost it more than $350m (£285m).