sales of milk in plastic bags have "sky-rocketed" at Sainsbury's since the supermarket rolled out the product nationally in February.

Britain's third-biggest grocer said sales of the two-pint bags, which use 75 per cent less packaging than standard plastic milk bottles, had soared to 110,000 a week. They now account for 10 per cent of all sales of two-pint packs of semi-skimmed milk – twice as many as Sainsbury's had forecast.

Emma Metcalf-King, the grocer's senior dairy buyer, said: "Sales have far exceeded our expectations."

The uplift in volumes of Sainbury's own-label milk in plastic bags could potentially usher in a new era in retailing of the liquid. Glass milk bottles were introduced by Express Dairies in 1880, although they have long since been overtaken by plastic cartons, and other countries, including Canada, China and India have used bags for milk for years. However, Waitrose scrapped a similar trial of milk in plastic bags earlier this year owing to disappointing sales. The commitment of the rest of the grocery industry is unclear.

Sainsbury's is considering rolling bags out across its milk ranges and in different sizes. It sells milk in a bag for 80p – 6p less than the same size milk carton – because it uses less packaging and is cheaper to produce.

The milk bags are part of a drive by Sainsbury's, which has 828 shops, to reduce its packaging by a third by 2015. It has estimated that switching to bags could save up to 1.4 million kilograms of plastic a year. It first introduced the bags in 2008 but introduced them in all of its stores in February.

Sainsbury's said the bags, which are made from strong, low density polyethylene, had been put through extensive testing to ensure durability, and no customers had complained about bags splitting. Customers buy milk bags for the first time with a reusable jug, which costs £1.98. Subsequently, shoppers just pop a bag in their trolley or basket. The bag fits into the Jugit container, which contains a spike that pierces the bag, forms a no-leak seal and pours milk through a spout.

To drive awareness, Sainsbury's plans to give away 400,000 free jugs next month. The grocer also gave 150,000 jugs to staff in February so they could explain usage to customers.

However, Waitrose phased out its milk bags a few months ago after launching them nationwide in 2007. A spokeswoman said: "We have taken the decision to stop selling milk in jugs and pouches as poor demand has led to high levels of milk being wasted.