Some 270 customers reported not getting their items by the next day.
Amazon must clarify that some items on its Prime service are not available for one-day delivery, the advertising regulator has ruled.
The online retailer is rebuked after wrongly claiming items on its Prime service were all eligible for one-day delivery.
The Advertising Standards Authority (ASA) said the online retailer’s promotion of a one-day delivery service for Amazon Prime members was “misleading” after around 270 people reported not getting their items by the next day.
The claim “must not appear again in its current form”, the ASA said, adding that shoppers were “likely to understand that, so long as they did not order too late or for Sunday delivery, all Prime labelled items would be available for delivery the next day with the one-day delivery option”.
Instead, that delivery option was not available for a “significant proportion of Prime labelled items”.
The ruling was obtained by The Times and is due to be announced later this week.
An Amazon spokesman said: “Amazon Prime offers fantastic benefits to members including one-day delivery on millions of eligible items at no extra cost.
“The expected delivery date is shown before an order is placed and throughout the shopping journey and we work relentlessly to meet this date.
“The overwhelming majority of one-day delivery orders are delivered when promised.
“A small proportion of orders missed the delivery promise last year during a period of extreme weather that impacted all carriers across the UK, and we provided support to impacted customers at the time.”
It is not the first time Amazon has fallen foul of advertising rules.
In April, four ads for electrical items were judged to be using “misleading” savings claims.
In August 2016, the company was ordered to clarify charges after being found to have misled shoppers over offers of free delivery on items that, when consumers checked out, became “free delivery in the UK on orders over £20”.
There have also been similar breaches in 2015, 2013 and 2012.
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