Forget the bogof. The number of buy-one-get-two-free deals has almost quadrupled year-on-year as retailers dig deeper than ever in the fight for market share.

Use of the multibuy deal increased from 36 to 132, an increase of 266%, with the sales mechanic seen in snacks, bakery, cakes & pastries, dairy, baby accessories, household goods, impulse lines, frozen and canned goods.

"These deals have increased phenomenally over the past two years," said Assosia director Kay Staniland. "This offer could work extremely well in categories where products can be stored for future use."

The mechanic, which was virtually unseen two years ago, is particularly prominent at Morrisons, accounting for 110, or 83% of the bogtf deals, a 300% increase in the year to date [Assosia 43w/e 31 October].

In response, Tesco this week massively increased its use of the promotion, with 35 buy-one-get-two-free deals hitting stores since late October [].

The big four have also ramped up the number of 'better than half price' savings on offer, according to promotion analyst Assosia, with the 2,374 deals displayed already [Assosia 43w/e 31 October] well ahead of last year's 12-month total of 2,132.

One promotions expert said Tesco and Morrisons were "buying growth" in order to hit sales targets or increase market share ahead of Christmas.

"These are the sorts of deals that make you cross the road and call your friends," he said. "I suspect Tesco are in a bit of a hole. Morrisons surprises me more but they're not doing as well as last year so maybe they're up against it as well."

Sales from the loss-leading deals would be enough to push zero or negative growth into positive territory, he added.

Aidan Bocci, chief executive of Commercial Advantage, said he expected the number of buy-one-get-two-free deals to increase well into next year.

"There was a brief respite in the past six months where consumers felt a bit more stability. But with the public sector job cuts and next year's VAT rise, people are now realising this is going to be a five-year struggle. Everyone is watching what they spend and becoming more and more focused on buying on deal."

He added: "If there's a general promotional escalation within two or three of the top four grocers, then others will have to follow."

One supermarket insider warned the heavy losses associated with some of the deals were unsustainable. "If we're not too careful we'll end up bankrupting ourselves," he said.

A Tesco spokesman said: "Tesco will offer the best value at a time when many households are looking to make savings. We recently announced £1.3bn in real value we're giving to customers this Christmas as a great way of saying thank you."

The big deals at the Big Four

Asda: Nescafé Cappuccino, 180g, £1 (was £2.40), 28/9/2010 to 21/10/10
Morrisons: Goodfella's thin crust pizzas, £2.49, buy-one-get-two-free, 1/11/10 to 7/11/10
Sainsbury's: Pringles, £2.03, three for the price of one, 29/09/10 to 3/10/10
Tesco: Cathedral City extra mature cheddar, 350g £4.50, buy-one-get-two-free, 3/11/2010 to 9/11/2010