The online retailer has faced accusations of poor working conditions and suppressing union support, but Amazon has suggested that it respects its employees' right to "join, form or not to join a labour union" of their choice.
In a letter to Jeff Bezos - the billionaire CEO of Amazon - Unite executive officer Sharon Graham wrote: "Although we do have members in Amazon, workers in your company are not currently free to join a union without fear and without obstruction and propaganda being deployed against them.
"So I am asking you to sign up to and abide by the declaration attached, which guarantees British and Irish workers the freedom to talk with and join unions without fear of retribution."
No UK Amazon warehouses are currently unionised, but the company insists it respects the right of its employees to join a union.
A spokesperson told the BBC: "Across Amazon, including in our fulfilment centres, we place enormous value on having daily conversations with each associate and work to make sure direct engagement with our employees is a strong part of our work culture.
"The fact is, we already offer excellent pay, excellent benefits and excellent opportunities for career growth, all while working in a safe, modern work environment. The unions know this."