Six high street carpet and furniture retailers are under investigation for using artificially high prices to exaggerate sales and price cuts.

The Office of Fair Trading (OFT) said it opened the investigation after finding that many retailers in the sector were misleading customers into thinking they were getting a bargain by artificially inflating the original price.

On Monday Tesco was fined £300,000 for using a similar practice to boost strawberry sales.

In the case of the furniture and carpet retailers, the OFT found "systematic" examples of artificially inflated reference pricing within the industry, through the use of "was" prices formerly charged by the retailer, "after-sale" prices that the trader intended to charge in the future, or recommended retail prices (RRPs) set by the manufacturer.

During monitoring of the six companies – which the OFT is not yet naming in the hope of reaching a speedy resolution with them – the overall average of sales of items at the reference price was just 5%.

The OFT said there were a significant number of products sold by some retailers where no sales at all took place at the artificially inflated price.

In all cases no explanation of how and when these higher prices were established were provided. The OFT has written to the six retailers asking them to stop using the pricing practices that mislead consumers, giving them until autumn to respond.

The OFT investigation comes ahead of the bank holiday weekend, a busy trading period for home furnishing retailers often marked by heavy sales promotions.

OFT director Gaucho Rasmussen said: "OFT research has found that reference pricing can mislead consumers into thinking the item they have bought is of higher value and quality, pressure them to buy there and then so they don't 'miss out' on the deal and also impair their judgment, as buying an item immediately means they do not get the chance to search the market for the real best deals.

"We have contacted a number of carpet and furniture retailers asking them to review their pricing practices and sign legally enforceable undertakings.

"This bank holiday sale season we would recommend that consumers ask sales staff when and for how long the reference price was used and also how many sales they achieved at this price. This will help them to determine whether they are getting a good deal."

Which? executive director Richard Lloyd said: "The OFT's warning sends a clear message to carpet and furniture stores that special offers really have to be special. It's unacceptable that shoppers are misled into thinking they're getting a good deal when that might not be the case."