Amazon EU has been found guilty of misleading consumers over the terms of its Amazon Prime delivery service.

The Advertising Standards Authority (ASA) made the ruling following three complaints about the "All You Can Eat One-Day" delivery system.

The complainants quoted text on the Amazon UK website that promised "Unlimited FREE One-Day Delivery on millions of eligible items sold by Amazon.co.uk" if users signed up for the premium Amazon Prime service.

Amazon promised express delivery for £4.49 per item and the membership listed free guaranteed one-day delivery on all eligible items delivered to mainland United Kingdom locations.

The three complaints all questioned whether that description was misleading because they found that a significant number of orders were not delivered within one day.

In its defence, Amazon stated that the Prime One-Day delivery was a guaranteed service and the web page did qualify the statement to say that customers would receive their order one business day after dispatch.

Amazon argued that when a Prime member placed an order they were provided with a guaranteed delivery date. This 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.

Amazon also gave details of a secret scheme it operates when Prime customers contact the customer services team.

In the event of a delivery not arriving by the guaranteed delivery date, buyers are compensated. However, this scheme is not published on the Amazon website and was applied on a case by case basis.

The ASA found against Amazon and upheld the complaints.

"Complainants had objected to claims, on the Prime web pages, that Prime members would enjoy a guaranteed One-Day Delivery service. We noted that neither these pages, nor the Prime terms and conditions page, explained that 'One-Day' meant one day after dispatch," the ASA's ruling said.

"Although we noted that the One-Day Delivery web page was accessible through a further link on the Prime terms and conditions page, we considered that Amazon had not made it sufficiently clear or prominent that 'One-Day Delivery' referred to one day after dispatch."

The ASA concluded that the Amazon UK website was misleading and that it breached the CAP Code of advertising for substantiation.

Amazon was told to include clear qualification on pages that used the phrase 'One-Day Delivery' to make it clear this referred to one day after dispatch.

The ASA also told the retailer to remove the claim that the service was guaranteed, until this could be substantiated.

United Kingdom