Amazon must clarify that some items on its Prime service are not available for next-day delivery after the advertising regulator ruled it was "misleading" consumers.
The online retail giant promoted its "one-day delivery" service on its website as part of the Amazon Prime membership, but around 270 people reported not receiving their delivery by the following day, the Advertising Standards Authority (ASA) said in a ruling obtained by The Times.
The ruling, due out this week, said that Amazon has been told to "make clear that some Prime labelled items were not available to be delivered by the next day", adding: "The ad must not appear again in its current form."
Explaining why they concluded the ad was "misleading", the ASA said in its ruling: "Because consumers 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, when a significant proportion of Prime labelled items were not available for delivery by the subsequent day with One Day Delivery, we concluded that the ad was misleading."
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."
This is not the first time Amazon has been found in breach of ASA rules.
In April, four ads for electronic items were banned for using "misleading" recommended retail prices (RRPs) and savings claims.
The ads, seen in July 2017 for a television, gaming monitor, laptop and electric toothbrush, all listed RRPs and savings of up to £300.
Each ad received a single complaint that the savings were misleading and unsubstantiated.
The ASA ruled that the ads must not appear again in their current form, adding: "We told Amazon to ensure that future references to RRPs reflected the price at which the products concerned were generally sold, and to ensure that they held adequate evidence to substantiate their savings claims."