You might find it a little bit busy in your local Threshers this weekend. A management plan to reward a select group of suppliers with discount vouchers misfired spectacularly after it was hijacked by hundreds of thousands of bargain hunters over the internet.

The off-licence chain admitted yesterday that its “40 per cent off” discount scheme had got completely out of hand after it was leaked on to the internet. More than a million vouchers have been downloaded and the Threshers website has crashed several times because of the huge number of customers looking for its nearest shop.

Discount schemes offered by other stores are also being hijacked by bargain hunters as the Christmas shopping season starts in earnest this weekend. Offers meant only for staff, loyal customers and specific groups of consumers are being openly swapped or sold on the internet.

Threshers had sent the voucher, which entitles the bearer to 40 per cent off up to £500 of wine and champagne, as a “thank you” to a selected group of suppliers. It also invited them to pass them on to “friends and family”. One voucher was placed on a blog of the South African winery Stormhoek last Saturday. It was quickly copied on to numerous other websites, blogs and chat rooms.

Last night Threshers confirmed that it would still honour every voucher. The offer runs until December 10 at 2,000 Threshers, Wine Rack, Bottoms Up, Haddows and The Local stores. Dirk Kind, the Threshers spokesman, said: “This has got way, way bigger than we thought it would. We sent it out to our suppliers and we included a bit saying, ‘Feel free to send it to your friends and family’. We have absolutely no idea how many vouchers have been downloaded.”

Staff at some stores are having to work up to four hours’ overtime to cope with demand. At the country’s smallest Threshers shop staff were struggling to cope with the demand yesterday. “It’s been a shock to the system, that’s for sure,” Oliver Fowler said at the off-licence in the City of London. “We’ve hardly had time to stock the shelves and the phones have hardly stopped ringing with people asking if the offer is for real.

“Champagne was particularly popular and the store has taken more than a hundred vouchers in the past two days.”

Near by, at another Central London Threshers, Phil Merrin, the manager, was busily replenishing shelves: “Christmas is a busy time of year for us anyway, but this has taken it up a notch. People are definitely buying in greater bulk.”

The deal has been compared to Hoover’s ill-fated offer in 1992 of a free return flight to New York for anyone spending more than £100. The company eventually had to spend more than £20 million on flights to the US to satisfy demand.

Although Threshers admitted that the sales promotion risked slashing its profit margins it pointed out that it had been offering a three-for-two offer on wine for more than a year — equivalent to a 33 per cent discount.

Robert Dirskovski, head of interactive media at the Direct Marketing Association, said: “It would appear that the Threshers scheme has got out of hand. There should have been a much tighter control mechanism. How much damage it will do the company depends on its profit margin but I would have thought that it would have welcomed the extra customers at this time of year.”

The Threshers vouchers are part of a trend among retailers to offer discount via e-mail. Selfridges, Gap, Habitat and Borders are among those offering up to a third off prices in the weeks before Christmas.

Many companies encourage recipients to pass on the vouchers to their friends as a form of “viral marketing” to attract new customers. It enables them to circumvent laws prohibiting companies from sending unsolicited commercial e-mails.

However, growing numbers of shoppers are now using the internet to track down the vouchers before they go to the high street or shop online.

Net vouchers

Money-off vouchers found on the internet:

Threshers 40% off champagne and wine until December 10

Selfridges & Co 20 per cent off until December 3

Habitat 25% off until December 3

Gap 30% off until December 3

Borders 20% off until December 3

Asda £10 when you spend £50 online until December 8

John Lewis £10 off when you spend over £50. No deadline given

Boots 10% off when spending £45 online. Until December 5

Marks & Spencer 10% off on all homeware. No deadline given

HMV 10% off CDs, Games, XBOX PSPs and DVDs. No deadline given

Early Learning Centre 10% off all online sales. No deadline given

Halfords 10% off online purchases until December 31

Source: Times research

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