The move has been widely condemned by brewers which fear the retailer has opened the floodgates for similar offers and year-round reductions.
Commenting on the 24-pack deal, a spokeswoman for Carlsberg-Tetley said: "Pricing is a retail decision, but this type of deep cutting is made with the store in mind and not the category. It is not what Carlsberg-Tetley wants, because we are trying to add value to the category."
She added: "We look to retailers to keep the value in the market."
Scottish Courage Brands' MD Keith Hogg said: "It's not just beer. Wine brands are also being heavily discounted. There is no point selling value-destructive volume."
He said Foster's, which last year entered the fray at £9.99 alongside similarly pitched rival Carling, was holding its price because the brewer had adopted a "tactical" pricing strategy.
But one industry source said the pricing could be good news for Carlsberg. "It might not be the price they want, but previously Carlsberg was nowhere. Now they have a platform and volume."
A spokesman for Somerfield said: "It's just one of our regular Megadeal offers.
"We always try to offer a lager, especially in the run-up to Christmas."