An Amazon payment page was misleading and could have led people to sign up for Amazon's subscription service, Prime, inadvertently, the UK's advertising watchdog has ruled.
Ten people complained to the ASA about this part of the check-out process.
The ASA said the page should not appear in its current form again and options related to signing up to Prime must be "presented clearly" in future.
Amazon said it was "disappointed" by the ruling.
In a statement, Amazon said: The evidence from millions of transactions demonstrates that customers have had positive experiences.
"The ASA has instead based its ruling on a handful of complaints and a subjective opinion of the page.
"We will continue our discussions with the ASA."
It said users had understood the options and only "a very small number of members" who signed up through the page had cancelled their membership within three days.
The "vast majority" had made active use of their membership, it said.
Amazon Prime costs £79 a year or £7.99 monthly. It offers one-day free delivery, as well as access to music, books and video streaming. It is often available with a free 30-day trial.
The page in question, seen in May and June 2019, included a gold box stating: "Order now with Prime." Directly beneath that, in a grey box, text stated: "Continue with free one-day delivery - pay later." Both these options signed users up to Prime.
In its ruling, the ASA said the presentation and wording of the two boxes had been "likely to be seen by the average consumer as separate options".
The option to not join Prime at all had been presented on the left-hand side of the page, had been smaller and "could easily be missed by consumers," it said.
"It was also in a faint colour - and compared to the option presented in the grey and gold boxes, it was significantly less prominent," the ruling said.
"We considered that the average consumer was likely to view the text within the grey and gold boxes as the only two options available, with the 'option' in the grey box allowing them to continue without signing up to Prime, when that was not the case."
Small print at the bottom of the page informed customers that after a 30-day free trial, they would be signed up to Amazon Prime.
In its response to the ASA, Amazon said it had periodically adjusted the way it had presented the two options on the page and used "customer satisfaction data to inform such adjustments and identify potential issues".
The retail giant said its primary objective was to make sure consumers who joined Prime did so intentionally and became active members who would make the most of the benefits of the subscription service.