Are they delivering the goods?The Office of Fair Trading has confirmed it is opening an investigation into the customer service of online supermarkets.
It is understood to be investigating customer complaints on a number of issues, including charging higher prices than are advertised on sites; charging for a premium product and supplying a regular item; charging more for online products than those in store; and using websites to offload food close to its sell-by date.
Most of these complaints were first raised by Which? magazine in 2001.
The OFT said in a statement: "We are examining supermarkets' pricing online. We have had confidential discussions with the supermarkets. This is an ongoing investigation so we cannot make any further comment."
A spokeswoman for Sainsbury's said the company could not comment as it was in consultation with the OFT. She added: "Customers can and do reject goods on delivery. The prices are the same as in store, including promotions.
"If we have to choose goods that are close to their 'best before' dates, this is pointed out to the customer on the doorstep and they can reject them. Where we have to make substitution, this is pointed out and it can be rejected. Refunds are made for rejections."
Tesco has said the goods it delivers are picked in local stores and therefore reflect the current price - and that customers may choose to have no substitutes when ordering. Asda has claimed it has the same prices nationwide in all its stores and for its online service.
Waitrose said neither it nor its Ocado subsidiary have been approached by the OFT.
Tesco is well on the way to selling £1bn a year online. In the first six months of this year it sold £307m online, up 27 per cent on the same period last year. And it reckons it fills 120,000 online orders a week.
Online supermarkets are a frequent subject of Which? reports. A recent survey of 1,500 shoppers conducted by the magazine found nine out of 10 users of the Sainsbury's website said their orders generally included substitutes, while three-quarters of those using Tesco said the same.
Along with the investigation of online operations, the OFT is looking into another aspect of the supermarket industry - generally regarded as one of the most competitive businesses in the UK - the supermarkets' relationship with their suppliers. A report on the topic is due out in January.