E-commerce giant Amazon has been warned by the UK's Advertising Standards Authority (ASA) it must clarify that some items on its Prime service are not eligible for next-day delivery.
The ASA said in a ruling, seen by the Times and due to be published next week, that Amazon has been "misleading" consumers by promoting one-day delivery as part of its premium Prime subscription service.
Around 270 people reported not receiving their delivery by the following day, leading the ASA to launch an investigation against the tech giant. The watchdog has now said the ad "must not appear again in its current form".
The ASA's ruling explained:
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 spokesperson 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."
Amazon previously found itself in hot water with the ASA earlier this year, when the watchdog banned four ads for electronic items for using misleading recommended retail prices to adjust savings claims.
The retailer was also reprimanded for similar infractions in 2015, 2013 and 2012.