Online retailer Amazon has been ordered to clarify advertising about its premium one-day delivery service after customers complained that they had been left waiting for orders.
The retailer advertised its Amazon Prime service with the promise of "free guaranteed one-day delivery" on all eligible items delivered to mainland UK addresses.
Three people complained that the offer was misleading because they had signed up to the service but found that a significant number of orders were not delivered within one day.
Defending the offer, Amazon said Prime customers were given a guaranteed delivery date when placing an order which would not necessarily be one day after the order was placed but would be one business day after the order was due to be dispatched.
The item might be sent on the day it was ordered or the following day, depending on the time the order was placed.
An order placed late on a Friday evening would be guaranteed to arrive on Monday, the next business day.
Upholding the complaints, the Advertising Standards Authority (ASA) said Amazon did not make it sufficiently clear that "one-day" meant one day after dispatch, and that consumers would understand from the claim that delivery would be on the day after the order was placed.
It noted that Amazon used couriers including Royal Mail's first class service for one-day deliveries which, according to the Royal Mail website, would take between one and two days.
The ASA said: "Because we understood that Amazon used non-guaranteed services such as Royal Mail first class for these orders, we considered that describing the service as 'guaranteed' was misleading.
"Because we considered the 'one-day delivery' claim was ambiguous and the claim that the service was 'guaranteed' was not adequately qualified or substantiated, we concluded that the website was misleading."
It told Amazon to include a clear qualification on pages which stated "one-day delivery" that this referred to one day after dispatch, and also told the company to remove the claim that the service was guaranteed until it could be substantiated.
An Amazon spokesman said: "Amazon measures success on its ability to deliver items by the delivery date that is provided to each customer when an order is placed.
"Whether it's Prime, Free Super Saver or any of our other delivery services, we work hard every day to ensure that our customers receive the products that they want, when they want them."