Amazon's one-day delivery ad for Prime members has been banned as misleading.
The UK advertising regulator said it had received 280 complaints, mostly from Prime customers who reported not receiving their packages within a day.
It said the ad "must not appear again in its current form" and Amazon must make clear that "a significant proportion" of Prime items were not available for next-day delivery.
Amazon said the "overwhelming majority" of one-day orders arrived on time.
It said the "period of extreme weather" last year meant "a small proportion of orders" missed their delivery deadline.
The firm's top service, Prime, offers next-day deliveries for £7.99 a month, or £79-a-year.
The Advertising Standards Authority said the adverts for Amazon Prime on its website in December meant customers would assume "one-day delivery" applied to all Prime-labelled items - and that these deliveries would arrive the day after the order was placed.
However, elsewhere on its website, Amazon explained that delivery time for its one-day service was "one business day after dispatch", and what time an order was placed would determine whether an item was dispatched on the same day.
The ASA said it was unlikely customers would find this information, before signing up for Amazon Prime.
"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," the ASA said in its ruling.
Amazon said the "vast majority" of the complaints followed media coverage of an initial handful of complaints.
A spokeswoman 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. "
Citizens Advice said online retailers should provide easy access to compensation if they fail to deliver items on time.
The consumer group said problems with late deliveries were not unique to Amazon.
Chief executive Gillian Guy said: "We've found 40% of people who used a premium delivery service received their parcel later than expected.
"It's more difficult for consumers to work out what they're owed when their parcels don't arrive on time if they've paid for a service like Amazon Prime, compared to when they pay for one-off deliveries.
