Workers at Amazon sites are "breaking bones" and having to be taken to hospital due to poor working conditions, a union is claiming.
GMB Union has staged protests outside Amazon's Swansea and Newport sites in recent years over working conditions. The site in Swansea is the second largest Amazon warehouse in Britain.
They are calling for Amazon to sit "round the table" to talk about how safety can be improved at its sites around the country.
Christopher Law, Site Leader at the Swansea warehouse said the allegations were "all hearsay" and not to "believe everything you hear".
In a damning statement, GMB Union claims "appalling" conditions at Amazon sites across the UK are causing workers to be seriously injured.
This comes after several protests outside Amazon warehouses in recent years and a Channel 4 documentary which criticised working conditions at the company's sites.
Mick Rix, Senior Organiser at GMB Union said some of the members who work for Amazon have broken bones, been "knocked unconscious" and had to be "taken away in ambulances".
There have been hundreds of ambulance call outs, pregnant women telling us they are forced to stand for ten hours a day, pick, stow, stretch and bend, pull heavy carts and walk miles - even miscarriages and pregnancy issues at work.
The Amazon site in Swansea is the company's second largest fulfilment centre in Britain. The 33 acre site is roughly the same size as 10 football pitches.
The workers there are preparing to fulfil millions of orders over the busy Christmas period, with a third of online purchases in the UK going through Amazon's website.
Christopher Law, Site Leader at the Swansea warehouse said the claims of "appalling" work conditions were not at all what he experiences at work.
When asked about claims of hundreds of ambulance call outs and employees being timed on their toilet breaks, Mr Law said that these were not things he recognised at the site and not something the company endorses.
Criticism has also been levelled at Amazon for allegations that workers must stow packages at a certain speed, causing physical problems and injuries with this being a particular problem at Christmas time.
However Mr Law said: "It is a busy time of year but we do scale to volume by hiring seasonal staff.
"Targets don't change throughout he year, our targets are set realistically.
"Like any other business we do have targets but they are achievable for all and that's how we measure.
"Essentially this time of year is like no other...we just make sure we're operating safely."
He added that there are always risks when working in a warehouse environment but his "focus will always be to make sure we're improving" on safety.
Dominic Hillman, a Pick Lead at the Swansea warehouse, has been working at Amazon for more than eleven years. He said safety and employee wellbeing is their top priority.
The guys have breaks, they have an hour break every day. We've got break up rooms...we've also got a new chill out room upstairs so they can unwind and relax. So safety is definitely our main focus and we do encourage everyone to report any incidences or potential incidents to management. >
Amazon operates more than 175 fulfilment centres across the world.
The company was launched in 1995 and more than 4,000 items are bought through the site each minute from sellers in the US alone.