They took off in the late 90s as an alternative to registered car dealerships with the promise of piling 'em high and selling 'em cheap.
But in the current climate of widespread falls in car prices the strain on car supermarkets is beginning to show and the jury is out on whether their advertised discounts are as good as they first appear.
Car supermarkets sell low price cars in a "no fuss" environment without the hardcore sales techniques associated with conventional manufacturer linked dealerships. They operate in direct competition with dealers, in a similar way to internet car importers such as www.jamjar.com, hoping to undercut them on price.
The logic goes that they can sell cheap because they either buy ex-company fleet cars at massive discounts, take surplus stock off UK manufacturers hands, or they import cheaper makes and models from the continent, passing on some of the savings to the buyer.
Go into a supermarket and the price on the car windscreen is what you'll be expected to pay. It is not a list price which can be negotiated down as with traditional dealerships.
The benefits are: what you see is what you get, you can pick up the keys and drive away on the same day, and although warranties may be shorter, most cars will still be under some form of manufacturer's guarantee when they are bought.
However, turning your back on the comfort zone of buying from a dealership with all the confidence brought by a strong after-sales service in favour of chasing a discount at a supermarket may end up costing more than you think.
For one thing, as the average price of cars has tumbled, the supermarkets' potential for offering really big savings has diminished. According to the Alliance and Leicester Car Price Index with What Car? magazine, the average cost of new cars has fallen by over 15% since 1998.
The supermarkets - along with the infamous "Rip off Britain" campaign and public uproar at the gap in prices between the UK and the rest of Europe - convinced manufacturers and dealers to cut prices. Ironically, the cheaper cars at dealerships have reduced the gap between list prices and deals on offer at supermarkets.
"Supermarkets have forced dealers to look at their prices, but the consequence is that if dealers do reduce prices, we have to react to that accordingly and this will have an effect, yes, on the deals we can offer," says Mark Thornber who runs ABC car supermarket in Salisbury and Andover.
"The prices of new cars have been coming down," says a spokesman for the Retail Motor Industry Federation, (RMI). "Car supermarkets probably didn't anticipate the falls in car prices and they are having to position themselves accordingly."
Paul McKenna, of Alliance car supermarket in Accrington, Lancashire, says: "The fall in the price of smaller cars in particular has had an effect on supermarket prices. It makes it more difficult to import and pass on the same level of savings as before. There is less room for movement."
There are no precise figures on how big a chunk of the market they account for, but supermarkets can be found across the country with most making their money from imported cars registered by the supermarkets, and sold as nearly new.
But don't expect to go in and get exactly what you are looking for. Unlike dealerships, they do not have access to a comprehensive list of makes and models.
But despite the new pressures on discounts, there is cash to be saved - even on smaller models.
Jobs & Money visited some supermarkets, such as Ignition in Burnley, Lancashire, to see exactly what we would pay for a 1.2 Fiat Punto with twin airbags, electric windows, central locking and power steering. There were plenty available for less than £6,000 - about £1,500 off list price.
But, the experts warn, such savings should not be taken at face value. "No doubt about it. There are deals still to be had and they are a very cheap way in many cases of getting a nearly new car," says David Motton of What Car? magazine. "But," he adds, "we always advise people to look behind the price. A lot of the cars don't arrive with UK specification and buyers could find themselves having to pay charges on top of the advertised price to comply. They may not come with immobilisers, for example, or alarms."
The face value discount doesn't take into account any potential saving off list price by negotiating with dealers either.
And there are numerous other hidden extra costs which could cut the initial saving even further.
Alliance's Paul McKenna says: "Not all supermarkets are the same. Some don't advertise on-the-road prices for example. This can be misleading.
"Some will add on charges for road tax, valeting and ad min and these can add up," he adds.
Mark Thornber of ABC says: "Buyers should find out about any additional, un-advertised costs. Some supermarkets will charge you £40 for a valet, for example, or even £10 a day to hold the car until you pick it up."
The no-frills supermarkets are not for everyone. But that doesn't mean they can't be used by any car buyer. As with internet importers touting cheap deals, at the very least they are worth checking for research purposes.
Finally, find out what price the car that you want is on offer for, and then use it to negotiate with a registered dealer - and you'll still be quids in.
