Amazon could be investigated by the Advertising Standards Authority (ASA) over complaints that its premium service is failing to deliver on time in the run-up to Christmas.
Though Amazon Prime claims it is offering ''unlimited one-day delivery'' customers contacted the advertising regulator said it was falling short of what it promised.
A spokeswoman for the ASA said, ''We have received a handful of complaints. We are considering whether to launch an investigation.''
Customers of the online retailer can choose free delivery above a minimum spend but are offered a free 30-day trial of its express delivery service, at the checkout counter, which costs £7.99 a month thereafter.
According to Amazon, the last order date for standard delivery is Wednesday but for Prime, which it says offers ''amazing delivery benefits on your Christmas shopping'', the last order date is Saturday.
Information elsewhere on the site says, ''Your order will be dispatched with the intention that it's delivered one day after dispatch.'' Customers are advised to contact customer services if they do not receive a parcel by the estimated delivery date.
The consumer rights group Which? said, ''If you paid for delivery by a certain date or time (eg by Christmas or next-day delivery) and the delivery arrives late, this is a breach of contract. If it was essential that your goods were delivered on time, you have the right to terminate the purchase and get a full refund.''
The ASA has, however, clarified that the number of complaints does not influence the likelihood of an investigation being launched.