Threshers, the off-licence, is being swamped by booze bargain-hunters after an online discount voucher that offers money off alcohol was downloaded by millions of internet users.

The retailer sent the voucher, which entitles the bearer to 40 per cent off up to £500-worth of wine and champagne, as a "thank you" to a selected group of suppliers and invited them to pass them on to "friends and family".

But once the voucher found its way onto the web, the promotion took on a life of its own. Stormhoek, a South African wine company, reported that the voucher had been downloaded 800,000 times from its site.

"This has got way, way bigger than we thought it would," a Thresher spokesman told Times Online. "To be honest, until we get some data in we have absolutely no idea how many vouchers have been downloaded, though we understand it was Google’s top hit today."

The Threshers site could not be accessed this afternoon.

The voucher has also been circulated by bloggers and is available from a slew of online message sites. Threshers has confirmed it will honour every voucher. The offer runs until December 10 at Threshers, Wine Rack, Bottoms Up, Haddows and The Local stores.

The sales promotion risks slashing Threshers’s profit margins, though the group has pointed out that it has been offering a three-for-two offer on wine for more than a year – equivalent to a 33 per cent discount. The site also stands to capture information on everybody who redeems a voucher - a hugely valuable commodity for retailers.

The Threshers vouchers are part of a growing trend among retailers to offer groups discount vouchers via e-mail. Selfridges and Gap have also issued e-vouchers, which are being forwarded between shoppers via e-mail ahead of Christmas.

Online bargain hunters are also congregating online to share news of the best bargains at sites such as hotdeals.com, which carries news of deals on products ranging from Dyson vacuum cleaners to Microsoft Xbox games consoles.

Some sites offer an alert system which informs users of the latest deals.

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